drm: mark drm_of_component_match_add dummy inline

The newly added drm_of_component_match_add helper is defined as
'static' in a header when CONFIG_OF is disabled, causing a warning
each time the header is included:

In file included from /git/arm-soc/drivers/gpu/drm/bridge/dw-hdmi.c:23:0:
include/drm/drm_of.h:33:13: error: 'drm_of_component_match_add' defined but not used [-Werror=unused-function]

This marks it 'inline' like the other such helpers in this file.

Fixes: 97ac0e47 ("drm: convert DT component matching to component_match_add_release()")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20161026085759.3875472-1-arnd@arndb.de