drm/i915/color: move check of gamma_enable to specific func/platform

Moved common code to check gamma_enable to specific funcs per platform
in bit_precision func. icl doesn't support that and chv has separate
enable knob for CGM LUT.

v2:
-Simplified chv_gamma_precision() [Ville]

Signed-off-by: Swati Sharma <swati2.sharma@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191009065542.27415-3-swati2.sharma@intel.com