Add check if palette register number is in correct range for few drivers
which miss it.  The regno value comes indirectly from user space.

Two drivers has converted check from BUG_ON() macro to just return an
error (non-zero value).
Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>