Implement simple blanking in pseudocolor modes for vt8500lcdfb

This follows the style of some other in-tree drivers by just setting
the hardware palette colors to all black. True Color modes are not
affected, but this at least allows to run xf86-video-fbdev without
errors due to blanking being unimplemented.
Signed-off-by: Alexey Charkov <alchark@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>