drm/udl: Make udl_enc_destroy() static

Fixes the following sparse warning:
drivers/gpu/drm/udl/udl_encoder.c:19:6: warning:
symbol 'udl_enc_destroy' was not declared. Should it be static?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/udl/udl_encoder.c

@@ -16,7 +16,7 @@
 #include "udl_drv.h"
 
 /* dummy encoder */
-void udl_enc_destroy(struct drm_encoder *encoder)
+static void udl_enc_destroy(struct drm_encoder *encoder)
 {
 	drm_encoder_cleanup(encoder);
 	kfree(encoder);