drm/i915/display: Add gamma precision function for CHV

intel_color_get_gamma_bit_precision() is extended for
cherryview by adding chv_gamma_precision(), i965 will use existing
i9xx_gamma_precision() func only.

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/1568030503-26747-2-git-send-email-swati2.sharma@intel.com