fb: JZ4770: Inline function jz4760fb_free_fb_info()

Also, call fb_dealloc_cmap() instead of fb_alloc_cmap() with an
empty color map, as this wasn't making any sense at all.
fb: JZ4770: Drop support for unsupported/untested pixel modes

This patch drops support for 1bpp, 2bpp, 4bpp and 8bpp modes,
as they are not available on the Foreground 1 layer.

It also drops 15bpp as it has never been tested.

Some code for 16bpp pixel mode has been kept because 16bpp
support will be added soonish.
fb: JZ4770: Tweak fb->var in fb_check_var() to something supported

The previous behaviour was to return -EINVAL if fb->var contained
values that didn't correspond to any modes proposed by the
framebuffer driver, while linux/fb.h explicitely specifies that
the fb_check_var callback should tweak 'var' to something
supported.
fb: JZ4770: Implement function jz4760fb_set_par() to switch modes
fb: JZ4770: Drop useless function jz4760fb_set_var()
fb: JZ4770: Enable support for 16bpp pixel mode.
fb: JZ4770: Don't set up DMA0 descriptor 0 as it is unused

It was set to transfer data from 'lcd_frame0' which has always
been an uninitialized pointer in the original code.
mips: JZ4770: Fix non-working macros in LCDC header