drm/ast: Reload gamma LUT after changing primary plane's color format

The gamma LUT has to be reloaded after changing the primary plane's
color format. This used to be done implicitly by the CRTC atomic_enable()
helper after updating the primary plane. With the recent reordering of
the steps, the primary plane's setup was moved last and invalidated
the gamma LUT. Fix this by setting the LUT from within atomic_flush().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Fixes: 2f0ddd89 ("drm/ast: Enable CRTC before planes")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dave Airlie <airlied@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200922144655.23624-1-tzimmermann@suse.de