drm/i915: Expose the legacy LUT via the GAMMA_LUT/GAMMA_LUT_SIZE props on gen2/3

drm/i915: Expose the legacy LUT via the GAMMA_LUT/GAMMA_LUT_SIZE props on gen2/3

Just so we don't leave gen2/3 out in the cold let's advertize the
legacy LUT via the GAMMA_LUT/GAMMA_LUT_SIZE props. Without the
GAMMA_LUT prop we can't actually load a LUT using the atomic ioctl
(in preparation for the day of 100% atomic driver).

Supposedly some gen2/3 platforms have an interpolated 10bit gamma mode
as well. It's slightly funkier than the i965+ mode since you have to
specify the slope for the interpolation by hand. But when I tried it
I couldn't get it to work, the hardware just insisted on using the
8bit more regardless of the state of the relevant PIPECONF bit.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190401200231.2333-7-ville.syrjala@linux.intel.com
Reviewed-by: Uma Shankar <uma.shankar@intel.com>