drm/vc4: Improve comments on vc4_plane_state members.

struct vc4_plane_state {

	struct drm_plane_state base;

	/* System memory copy of the display list for this element, computed

	 * at atomic_check time.

	 */

	u32 *dlist;

	u32 dlist_size; /* Number of dwords in allocated for the display list */

	u32 dlist_size; /* Number of dwords allocated for the display list */

	u32 dlist_count; /* Number of used dwords in the display list. */

	/* Offset in the dlist to pointer word 0. */

	u32 pw0_offset;

	/* Offset where the plane's dlist was last stored in the

	   hardware at vc4_crtc_atomic_flush() time.

	 * hardware at vc4_crtc_atomic_flush() time.

	 */

	u32 *hw_dlist;

};