drm/i915: Fix return type of mode_valid function hook

All of the functions used for intel_dvo_dev_ops.mode_valid have a return
type of enum drm_mode_status, but the mode_valid field in the struct
definition has a return type of int.

The mismatched return type breaks forward edge kCFI since the underlying
function definitions do not match the function hook definition.

The return type of the mode_valid field 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
Cc: llvm@lists.linux.dev
Signed-off-by: Nathan Huckleberry <nhuck@google.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220913205531.155046-1-nhuck@google.com

drivers/gpu/drm/i915/display/intel_dvo_dev.h

@@ -75,8 +75,8 @@ struct intel_dvo_dev_ops {
 	 *
 	 * \return MODE_OK if the mode is valid, or another MODE_* otherwise.
 	 */
-	int (*mode_valid)(struct intel_dvo_device *dvo,
-			  struct drm_display_mode *mode);
+	enum drm_mode_status (*mode_valid)(struct intel_dvo_device *dvo,
+					   struct drm_display_mode *mode);
 
 	/*
 	 * Callback for preparing mode changes on an output