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 <[email protected]>
Signed-off-by: Ville Syrjälä <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Uma Shankar <[email protected]>

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