drm: prefer inline over __inline__

Remove last users of __inline__.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180327204722.31246-1-jani.nikula@intel.com