[PATCH] nvidiafb: Add flat panel dither support

nvidiafb didn't fully hook-up the code it borrowed from X for doing flat
panel dithering (this is useful for 6 bits panels).  This adds a driver
option to force it, and by default "reads" the current value from the chip
to get the firmware setting.  It significantly improves the quality of
images on the iMac G5 I have here (though the X driver doesn't yet "read"
the current value and defaults to 0, so you have to add Option "FBDither"
"true" to your X config file to get that, I'll try to fix X.org to "read"
the default unless specified asap).

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: "Antonino A. Daplas" <adaplas@hotpop.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>