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