fbdev: Improve performance of cfb_imageblit()

Improve the performance of cfb_imageblit() by manually unrolling
the inner blitting loop and moving some invariants out. The compiler
failed to do this automatically. This change keeps cfb_imageblit()
in sync with sys_imagebit().

A microbenchmark measures the average number of CPU cycles
for cfb_imageblit() after a stabilizing period of a few minutes
(i7-4790, FullHD, simpledrm, kernel with debugging).

cfb_imageblit(), new: 15724 cycles
cfb_imageblit(): old: 30566 cycles

In the optimized case, cfb_imageblit() is now ~2x faster than before.

v3:
	* fix commit description (Pekka)

Signed-off-by: Thomas Zimmermann <[email protected]>
Reviewed-by: Javier Martinez Canillas <[email protected]>
Acked-by: Sam Ravnborg <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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