drm/dbi: Use a static inline stub for mipi_dbi_debugfs_init()

Replace the slightly odd "#define <function> NULL" thing with
a standard static inline stub.

Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211221193754.12287-2-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>

include/drm/drm_mipi_dbi.h

@@ -194,7 +194,7 @@ int mipi_dbi_buf_copy(void *dst, struct drm_framebuffer *fb,
 #ifdef CONFIG_DEBUG_FS
 void mipi_dbi_debugfs_init(struct drm_minor *minor);
 #else
-#define mipi_dbi_debugfs_init		NULL
+static inline void mipi_dbi_debugfs_init(struct drm_minor *minor) {}
 #endif
 
 #endif /* __LINUX_MIPI_DBI_H */