pm2fb: RDAC_WR barriers clean up

This patch fixes a possible barriers problem in RDAC_WR functions, then
remove redudant ones.

It also removes one unneeded register load from blitter function().

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