drm/meson: Fix return type of meson_encoder_cvbs_mode_valid()

With clang's kernel control flow integrity (kCFI, CONFIG_CFI_CLANG),
indirect call targets are validated against the expected function
pointer prototype to make sure the call target is valid to help mitigate
ROP attacks. If they are not identical, there is a failure at run time,
which manifests as either a kernel panic or thread getting killed. A
proposed warning in clang aims to catch these at compile time, which
reveals:

drivers/gpu/drm/meson/meson_encoder_cvbs.c:211:16: 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 = meson_encoder_cvbs_mode_valid,
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

->mode_valid() in 'struct drm_bridge_funcs' expects a return type of
'enum drm_mode_status', not 'int'. Adjust the return type of
meson_encoder_cvbs_mode_valid() to match the prototype's to resolve the
warning and CFI failure.

Link: https://github.com/ClangBuiltLinux/linux/issues/1750
Reported-by: Sami Tolvanen <[e-mail]>
Signed-off-by: Nathan Chancellor <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Signed-off-by: Neil Armstrong <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/meson/meson_encoder_cvbs.c

···
 	return i;
 }
 
-static int meson_encoder_cvbs_mode_valid(struct drm_bridge *bridge,
-					const struct drm_display_info *display_info,
-					const struct drm_display_mode *mode)
+static enum drm_mode_status
+meson_encoder_cvbs_mode_valid(struct drm_bridge *bridge,
+			      const struct drm_display_info *display_info,
+			      const struct drm_display_mode *mode)
 {
 	if (meson_cvbs_get_mode(mode))
 		return MODE_OK;