staging: sm750fb: add common function to set color offsets and visual mode

This patch will unify code for updating color offsets and frame buffer
visual mode.

Signed-off-by: Matej Dujava <mdujava@kocurkovo.cz>
Link: https://lore.kernel.org/r/1588277366-19354-3-git-send-email-mdujava@kocurkovo.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>