G2D: Fallback to NEON optimized CPU backend for unsupported blits

The G2D driver only supports framebuffer->framebuffer blits and
also can't be used to accelerate dragging windows to the right
(without hacking the kernel driver to do two-pass blit there).
This patch adds fallback to NEON optimized CPU backend instead
of resorting to use poorly performing fbBlt in these cases.

Note: we assume that ioctls normally do not fail (even if they
      do, the slow old style fallback to fbBlt is not the worst
      thing to worry about).

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Enable G2D acceleration by default on sun4i hardware

With the fallback to CPU backend for unsupported blits and also
threshold for avoiding small blits, now G2D should always provide
best overall performance.

The users of recent versions of xf86-video-sunxifb are supposed
to also have a reasonably recent version of linux-sunxi kernel.
Which includes the following fix:
  https://github.com/linux-sunxi/linux-sunxi/commit/3d49345343a1535b

The users of old kernels are going to see screen corruption on
dragging windows and scrolling. They just should upgrade :)

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>