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