drm/i915: Refactor icl_is_hdr_plane

Change the api in order to enable callers that can't supply a valid
intel_plane pointer, as would be the case prior to calling
drm_universal_plane_init.

v4:
- Rename variables and move a declaration (Ville)

v6:
- Rebase and fix merge conflict

Cc: Uma Shankar <uma.shankar@intel.com>
Cc: Shashank Sharma <shashank.sharma@intel.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Kevin Strasser <kevin.strasser@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1552437513-22648-3-git-send-email-kevin.strasser@intel.com