drm: Add NULL check about irq functions

The struct drm_driver has some function pointers for irq. They are
gpu specific and some functions aren't essential things. This can
prevents creation of unnecessary dummy function for irq.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>