drm/i915/display: Add func to get gamma bit precision

Each platform supports different gamma modes and each gamma mode
has different bit precision. Here bit precision corresponds
to number of bits the hw LUT supports.

Add func per platform to return bit precision corresponding to gamma mode
which will be later used as a parameter in lut comparison function
intel_color_lut_equal().

This is done for legacy, ilk, glk and their variant platforms.

v6:  -Added func intel_color_get_bit_precision() to get bit precision for
      gamma and degamma lut readout depending upon platform and
      corresponding to load_luts() [Ankit]
     -Made patch11 as patch3 [Jani]
v7:  -Renamed func intel_color_get_bit_precision() to
      intel_color_get_gamma_bit_precision()
     -Added separate function/platform for gamma bit precision [Ville]
     -Corrected checkpatch warnings
v8:  -Split patch 3 into 4 separate patches
v9:  -Changed commit message, gave more info [Uma]
     -Added precision func for icl+ platform
v10: -Removed precision func for chv and icl+ platforms [Jani]
     -Added gamma_enable check once [Jani]

Signed-off-by: Swati Sharma <swati2.sharma@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1567538578-4489-2-git-send-email-swati2.sharma@intel.com