cirrusfb: acceleration improvements

- Fix color expansion for 16bpp and 32bpp modes in the
  cirrusfb_RectFill().

- Make a function with a common blitter code (cirrusfb_set_blitter).

- Add fb_sync function to allow a higher layer synchronize with the
  blitter.

- Kill one redundant blitter reset.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>