drm: add proper return value for drm_mode_crtc_set_gamma_size

drm_mode_crtc_set_gamma_size returns boolean true for success
and false for failure. This is not very kernel conform, so
change it to return 0 for success and a propert error code
otherwise. Noone checks the return value, so no users have to
be fixed.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>