drm/amd/amdgpu/dce_v11_0: Supply description for function param 'async'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/amd/amdgpu/dce_v11_0.c:255: warning: Function parameter or member 'async' not described in 'dce_v11_0_page_flip'

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Luben Tuikov <luben.tuikov@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>