drm/i915/display: Add HDR mode helper function

Add helper function with returns if HDR mode in on

Signed-off-by: Tejas Upadhyay <tejaskumarx.surendrakumar.upadhyay@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210907113658.1351456-1-tejaskumarx.surendrakumar.upadhyay@intel.com


[vsyrjala: fix up alignment to match codingstyle]
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>