drm/omap: use degamma property for gamma table

omapdrm supports gamma via GAMMA_LUT property. However, the HW we have
is:

gamma -> ctm -> out

instead of what the model DRM framework uses:

ctm -> gamma -> out

As the following patches add CTM support for omapdrm, lets first fix the
gamma.

This patch changes the property from GAMMA_LUT to DEGAMMA_LUT, and thus
we will have:

degamma -> ctm -> out

and the legacy ioctl will continue working as before.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201103080310.164453-3-tomi.valkeinen@ti.com