drm/radeon/radeon_device: Provide function name in kernel-doc header

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

 drivers/gpu/drm/radeon/radeon_device.c:1101: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst

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
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_device.c

@@ -1098,7 +1098,8 @@ static bool radeon_check_pot_argument(int arg)
 }
 
 /**
- * Determine a sensible default GART size according to ASIC family.
+ * radeon_gart_size_auto - Determine a sensible default GART size
+ *                         according to ASIC family.
  *
  * @family: ASIC family name
  */