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: 1baf912 ("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

drivers/gpu/drm/mgag200/mgag200_mode.c

@@ -640,6 +640,11 @@ void mgag200_crtc_helper_atomic_enable(struct drm_crtc *crtc, struct drm_atomic_
 	if (funcs->pixpllc_atomic_update)
 		funcs->pixpllc_atomic_update(crtc, old_state);
 
+	if (crtc_state->gamma_lut)
+		mgag200_crtc_set_gamma(mdev, format, crtc_state->gamma_lut->data);
+	else
+		mgag200_crtc_set_gamma_linear(mdev, format);
+
 	mgag200_enable_display(mdev);
 
 	if (funcs->enable_vidrst)