udl-kms: avoid division

Division is slow, so it shouldn't be done by the pixel generating code.
The driver supports only 2 or 4 bytes per pixel, so we can replace
division with a shift.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>