drm: automatic legacy gamma support

To support legacy gamma ioctls the drivers need to set
drm_crtc_funcs.gamma_set either to a custom implementation or to
drm_atomic_helper_legacy_gamma_set. Most of the atomic drivers do the
latter.

We can simplify this by making the core handle it automatically.

Move the drm_atomic_helper_legacy_gamma_set() functionality into
drm_color_mgmt.c to make drm_mode_gamma_set_ioctl() use
drm_crtc_funcs.gamma_set if set or GAMMA_LUT property if not.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201211114237.213288-2-tomi.valkeinen@ti.com