drm/i915: define _MMIO_PLANE() in terms of _PLANE() not _MMIO_PIPE()

Minor semantic nit, no functional changes.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181031110453.12722-2-jani.nikula@intel.com