drm/mgag200: Fix gamma lut not initialized for G200ER, G200EV, G200SE

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.
This has been fixed with commit ad81e234 ("drm/mgag200: Fix gamma
lut not initialized.") for most G200, but G200ER, G200EV, G200SE use
their own version of crtc_helper_atomic_enable() and need to be fixed
too.

Fixes: 1baf9127

 ("drm/mgag200: Replace simple-KMS with regular atomic helpers")
Cc: <stable@vger.kernel.org> #v6.1+
Reported-by: Roger Sewell <roger.sewell@cantab.net>
Suggested-by: Roger Sewell <roger.sewell@cantab.net>
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20231214163849.359691-1-jfalempe@redhat.com