drm: Make syncobj import/export functions static

Fix the following sparse warnings by making functions static:
drm_syncobj.c:420:5: warning: symbol 'drm_syncobj_import_sync_file_fence' was not declared. Should it be static?
drm_syncobj.c:441:5: warning: symbol 'drm_syncobj_export_sync_file' was not declared. Should it be static?

Cc: Dave Airlie <airlied@redhat.com>
Fixes: 3ee45a3b533a ("drm/syncobj: add sync_file interaction. (v1.2)")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170901165328.24459-1-ville.syrjala@linux.intel.com
Reviewed-by: Thierry Reding <treding@nvidia.com>

drivers/gpu/drm/drm_syncobj.c

@@ -275,8 +275,8 @@ static int drm_syncobj_fd_to_handle(struct drm_file *file_private,
 	return 0;
 }
 
-int drm_syncobj_import_sync_file_fence(struct drm_file *file_private,
-				       int fd, int handle)
+static int drm_syncobj_import_sync_file_fence(struct drm_file *file_private,
+					      int fd, int handle)
 {
 	struct dma_fence *fence = sync_file_get_fence(fd);
 	struct drm_syncobj *syncobj;
@@ -296,8 +296,8 @@ int drm_syncobj_import_sync_file_fence(struct drm_file *file_private,
 	return 0;
 }
 
-int drm_syncobj_export_sync_file(struct drm_file *file_private,
-				 int handle, int *p_fd)
+static int drm_syncobj_export_sync_file(struct drm_file *file_private,
+					int handle, int *p_fd)
 {
 	int ret;
 	struct dma_fence *fence;