Optimize kernel cfbfillrect function

This is a  patch to speed up the kernel
cfbfillrect function used for clearing the screen or filling an area
with a solid color.

The previous implementation was doing reads from the uncached framebuffer,
in certain circumstances, even for simple fills, which for performance
reasons, on most platforms, is ill-advised unless strictly necessary to
conform to the raster operation. In these circumstances, doing 64-bit
access on 64-bit systems does not serve much purpose except for probing
for corner-case bugs and race conditions in the hardware's framebuffer
bus implementation. On all systems, for 16bpp, it is better to take
advantage of write-combining features of the framebuffer and write a
half-word if required for the left and right edges.

Additionally, on most platforms, integer divides are relatively slow so are
best avoided, especially in inner loops.

This patch implements optimizations for 16bpp and 32bpp that avoid
framebuffer read operations during cfbfillrect if the raster-operation
is GXcopy. This applies to both 32-bit (BITS_PER_LONG) and 64-bit
systems (BITS_PER_LONG == 64). Although cfbfillrect is not usually a
bottleneck for the console text framebuffer (because regions filled
usually are aligned to a multiple of 2 or 4 x-coordinate and reads
are avoided), in some use-cases this patch could deliver a significant
speed-up.

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>
Speed up console framebuffer imageblit function

Especially on platforms with a slower CPU but a relatively high
framebuffer fill bandwidth, like current ARM devices, the existing
console monochrome imageblit function used to draw console text is
suboptimal for common pixel depths such as 16bpp and 32bpp. The existing
code is quite general and can deal with several pixel depths. By creating
special case functions for 16bpp and 32bpp, by far the most common pixel
formats used on modern systems, a significant speed-up is attained
which can be readily felt on ARM-based devices like the Raspberry Pi
and the Allwinner platform, but should help any platform using the
fb layer.

The special case functions allow constant folding, eliminating a number
of instructions including divide operations, and allow the use of an
unrolled loop, eliminating instructions with a variable shift size,
reducing source memory access instructions, and eliminating excessive
branching. These unrolled loops also allow much better code optimization
by the C compiler. The code that selects which optimized variant is used
is also simplified, eliminating integer divide instructions.

The speed-up, measured by timing 'cat file.txt' in the console, varies
between 40% and 70%, when testing on the Raspberry Pi and Allwinner
ARM-based platforms, depending on font size and the pixel depth, with
the greater benefit for 32bpp.

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>