drm/amd/amd_shared.h: Add missing doc for PP_GFX_DCS_MASK

This symbol is missing documentation:

	drivers/gpu/drm/amd/include/amd_shared.h:224: warning: Enum value 'PP_GFX_DCS_MASK' not described in enum 'PP_FEATURE_MASK'

Document it.

Fixes: 680602d6 ("drm/amd/pm: enable DCS")
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>