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>

drivers/gpu/drm/amd/include/amd_shared.h

@@ -193,6 +193,7 @@ enum amd_powergating_state {
  * @PP_ACG_MASK: Adaptive clock generator.
  * @PP_STUTTER_MODE: Stutter mode.
  * @PP_AVFS_MASK: Adaptive voltage and frequency scaling.
+ * @PP_GFX_DCS_MASK: GFX Async DCS.
  *
  * To override these settings on boot, append amdgpu.ppfeaturemask=<mask> to
  * the kernel's command line parameters. This is usually done through a system's