drm/omapdrm: Implement gamma_lut atomic crtc properties

Implement gamma_lut atomic crtc properties, set crtc gamma size to 256
for all crtcs and use drm_atomic_helper_legacy_gamma_set() as
gamma_set func. The tv-out crtc has 1024 element gamma table (with
10bit precision) in HW, but current Xorg server does not accept
anything else but 256 elements so that is used for all CRTCs. The dss
dispc API converts table of any length for HW and uses linear
interpolation in the process. The default gamma table is restored
if gamma_lut property is deleted.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>