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.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>