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>