drm/radeon/evergreen: Add comment for 'evergreen_page_flip()'s 'async' param

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

 drivers/gpu/drm/radeon/evergreen.c: In function ‘evergreen_gpu_init’:
 drivers/gpu/drm/radeon/evergreen.c:1419: warning: Function parameter or member 'async' not described in 'evergreen_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: 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>