[ARM] pxafb: add palette format support for LCCR4_PAL_FOR_3

Add the palette format support for LCCR4_PAL_FOR_3, and fix the
issue of LCCR4 being never assigned.

Also remove the useless pxafb_set_truecolor().

Signed-off-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Eric Miao <ycmiao@ycmiao-hp520.(none)>