drm/i915: un-inline icl_hdr_plane_mask() to simplify includes

This lets us drop the heavy intel_display.h include from intel_sprite.h.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/16e13b6f207f52f8810a06f71a08e637f6397dc8.1667383630.git.jani.nikula@intel.com