drm/mgag200: Fix gamma lut not initialized.

When mgag200 switched from simple KMS to regular atomic helpers,
the initialization of the gamma settings was lost.
This leads to a black screen, if the bios/uefi doesn't use the same
pixel color depth.

v2: rebase on top of drm-misc-fixes, and add Cc stable tag.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=2171155
Fixes: 1baf9127

 ("drm/mgag200: Replace simple-KMS with regular atomic helpers")
Cc: <stable@vger.kernel.org>
Tested-by: Phil Oester <kernel@linuxace.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230510131034.284078-1-jfalempe@redhat.com