drm/i915: move is_ccs_modifier to an inline

There is no need for this to be out of line.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/bb73a151b7b780f927edeb7e121449446592805d.1612536383.git.jani.nikula@intel.com