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 <[email protected]>
Signed-off-by: Antonino Daplas <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>