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

drivers/gpu/drm/i915/i915_reg.h

@@ -160,7 +160,7 @@ static inline bool i915_mmio_reg_valid(i915_reg_t reg)
 #define _PIPE(pipe, a, b) _PICK_EVEN(pipe, a, b)
 #define _MMIO_PIPE(pipe, a, b) _MMIO(_PIPE(pipe, a, b))
 #define _PLANE(plane, a, b) _PICK_EVEN(plane, a, b)
-#define _MMIO_PLANE(plane, a, b) _MMIO_PIPE(plane, a, b)
+#define _MMIO_PLANE(plane, a, b) _MMIO(_PLANE(plane, a, b))
 #define _TRANS(tran, a, b) _PICK_EVEN(tran, a, b)
 #define _MMIO_TRANS(tran, a, b) _MMIO(_TRANS(tran, a, b))
 #define _PORT(port, a, b) _PICK_EVEN(port, a, b)