drm/i915: s/plane/i9xx_plane/

Call the enum i9xx_plane_id variable i9xx_plane like we do elsewhere.

Cc: Hans de Goede <j.w.r.degoede@gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180130203807.13721-7-ville.syrjala@linux.intel.com


Reviewed-by: Mika Kahola <mika.kahola@intel.com>