drm: xlnx: Fix return type of zynqmp_dp_bridge_mode_valid

The mode_valid field in drm_bridge_helper_funcs is expected to be of
type
enum drm_mode_status (* mode_valid) (struct drm_bridge *bridge,
                                     struct drm_display_mode *mode);

The mismatched return type breaks forward edge kCFI since the underlying
function definition does not match the function hook definition. A new
warning in clang will catch this at compile time:

  drivers/gpu/drm/xlnx/zynqmp_dp.c:1573:16: error: incompatible function pointer types initializing 'enum drm_mode_status (*)(struct drm_bridge *, const struct drm_display_info *, const struct drm_display_mode *)' with an expression of type 'int (struct drm_bridge *, const struct drm_display_info *, const struct drm_display_mode *)' [-Werror,-Wincompatible-function-pointer-types-strict]
          .mode_valid = zynqmp_dp_bridge_mode_valid,
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~
  1 error generated.

The return type of zynqmp_dp_bridge_mode_valid should be changed from
int to enum drm_mode_status.
Reported-by: Dan Carpenter <error27@gmail.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/1703
Link: https://github.com/ClangBuiltLinux/linux/issues/1750Signed-off-by: Nathan Huckleberry <nhuck@google.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
[nathan: Rebase on drm-misc-next and fix conflicts
         Add note about new clang warning]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

drivers/gpu/drm/xlnx/zynqmp_dp.c

@@ -1362,9 +1362,10 @@ static void zynqmp_dp_bridge_detach(struct drm_bridge *bridge)
 	zynqmp_dp_aux_cleanup(dp);
 }
 
-static int zynqmp_dp_bridge_mode_valid(struct drm_bridge *bridge,
-				       const struct drm_display_info *info,
-				       const struct drm_display_mode *mode)
+static enum drm_mode_status
+zynqmp_dp_bridge_mode_valid(struct drm_bridge *bridge,
+			    const struct drm_display_info *info,
+			    const struct drm_display_mode *mode)
 {
 	struct zynqmp_dp *dp = bridge_to_dp(bridge);
 	int rate;