pm3fb: various fixes

 - fix 16-bit RGBA 4444 and 5551 modes (use fbset -rgba to set them)
 - kill wmb() after each register write by PM3_SLOW_WRITE_REG
 - kill inaccurate comments (leftovers from skeletonfb.c)
 - kill PCI class restriction, now it is more general and shorter
 - white spaces and other minor fixes and source reducing

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>