video: fbdev: pmag-ba-fb: Optimize Bt455 colormap addressing

Use the address autoincrement feature when accessing successive palette
entries and also skip loading a palette address in overlay register
assesses which do not use that address.  Provide a red/green/blue
register sequencer reset helper for use in overlay register assesses
where the state of the sequencer is not known.

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>