video: fbdev: pmag-ba-fb: Fix and rework Bt455 colormap handling

The Bt455 is a greyscale RAMDAC, using the green color palette entries
only while still providing registers for the red and blue components,
all the three of which have to be loaded on palette updates.  Chip
documentation [1] mandates that the unused red and blue registers are
written with 0.

Therefore update code to follow this requirement and given that it makes
the red and blue components unusable remove them from internal API calls
altogether.

References:

[1] "Bt454 Bt455 170 MHz Monolithic CMOS 16 Color Palette RAMDAC",
    Brooktree Corporation, Document Number: L454001, Rev. I

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>