drm/amd/amdgpu/amdgpu_vram_mgr: Add missing descriptions for 'dev' and 'dir'

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

 drivers/gpu/drm/amd/amdgpu/amdgpu_vram_mgr.c:648: warning: Function parameter or member 'dev' not described in 'amdgpu_vram_mgr_free_sgt'
 drivers/gpu/drm/amd/amdgpu/amdgpu_vram_mgr.c:648: warning: Function parameter or member 'dir' not described in 'amdgpu_vram_mgr_free_sgt'

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>

drivers/gpu/drm/amd/amdgpu/amdgpu_vram_mgr.c

@@ -637,6 +637,8 @@ int amdgpu_vram_mgr_alloc_sgt(struct amdgpu_device *adev,
  * amdgpu_vram_mgr_free_sgt - allocate and fill a sg table
  *
  * @adev: amdgpu device pointer
+ * @dev: device pointer
+ * @dir: data direction of resource to unmap
  * @sgt: sg table to free
  *
  * Free a previously allocate sg table.