drm/doc: Drop kerneldoc for static functions in drm_irq.c

At least in drm core we only document the driver interfaces using
kerneldoc. For internals an unstructured comment is good enough.

Fixes a warning from kernel-doc, too.

Acked-by: Liviu Dudau <Liviu.Dudau@arm.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1464729075-22243-5-git-send-email-daniel.vetter@ffwll.ch