drm/i915: Constify intel_drrs_init() args

Pass the fixed_mode as const to intel_drrs_init() since it's
not supposed to mutate the mode.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220310004802.16310-3-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>