drm/radeon/radeon_ib: Supply description for 'radeon_ib_get's get param

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

 drivers/gpu/drm/radeon/radeon_ib.c:61: warning: Function parameter or member 'vm' not described in 'radeon_ib_get'

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>