drm/radeon/ni: Demote vague attempt at function header doc

drm/radeon/ni: Demote vague attempt at function header doc

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

 drivers/gpu/drm/radeon/ni.c: In function ‘cayman_gpu_init’:
 drivers/gpu/drm/radeon/ni.c:2679: warning: Function parameter or member 'ring' not described in 'cayman_vm_flush'
 drivers/gpu/drm/radeon/ni.c:2679: warning: Function parameter or member 'vm_id' not described in 'cayman_vm_flush'
 drivers/gpu/drm/radeon/ni.c:2679: warning: Function parameter or member 'pd_addr' not described in 'cayman_vm_flush'

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>