drm: Add missing static storage class specifier in drm_irq.c file

Fixes the following sparse warning:
drivers/gpu/drm/drm_irq.c:1239:6:
warning: symbol 'drm_handle_vblank_events' was not declared. Should it be static?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>