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

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

commit 11f9eb899ecc8c02b769cf8d2532ba12786a7af7 upstream.

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 ad81e23426a6 ("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: 1baf9127c482 ("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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>