drm/i915/sprite: un-inline icl_is_hdr_plane()

Avoid including the i915_drv.h mega header from other header files to
make further header cleanup easier.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/5f998c72018c0f1cc5cdb239327a1281d21f4c0e.1565085691.git.jani.nikula@intel.com