drm: drm_dsc.h: fix a kernel-doc markup

As warned by Sphinx:

	./Documentation/gpu/drm-kms-helpers:305: ./include/drm/drm_dsc.h:587: WARNING: Unparseable C cross-reference: 'struct'
	Invalid C declaration: Expected identifier in nested name, got keyword: struct [error at 6]
	  struct
	  ------^

The markup for one struct is wrong, as struct is used twice.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/3d467022325e15bba8dcb13da8fb730099303266.1601467849.git.mchehab+huawei@kernel.org

include/drm/drm_dsc.h

@@ -588,7 +588,7 @@ struct drm_dsc_picture_parameter_set {
  * This structure represents the DSC PPS infoframe required to send the Picture
  * Parameter Set metadata required before enabling VESA Display Stream
  * Compression. This is based on the DP Secondary Data Packet structure and
- * comprises of SDP Header as defined &struct struct dp_sdp_header in drm_dp_helper.h
+ * comprises of SDP Header as defined &struct dp_sdp_header in drm_dp_helper.h
  * and PPS payload defined in &struct drm_dsc_picture_parameter_set.
  *
  * @pps_header: Header for PPS as per DP SDP header format of type