drm/i915: Program plane gamma ramps

All sprite planes have a progammable gamma ramp. Set it up with
a linear ramp on all platforms. This actually matches the reset
value but soon we'll want to reprogram this ramp on some machines,
so let's just set it up across the board.

Note that on pre-IVB the hardware bypasses the gamma unit
unless a YCbCr pixel format is used.

v2: Add parens around << in ilk_linear_gamma()
    Skip gamma programming for RGB on pre-IVB
    s/DVSGAMC/DVSGAMC_G4X/

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/20190703200824.5971-4-ville.syrjala@linux.intel.com