drm/i915: Check for driver readyness before handling an underrun interrupt

When we takeover from the BIOS and install our interrupt handler, the
BIOS may have left us a few surprises in the form of spontaneous
interrupts. (This is especially likely on hardware like 965gm where
display fifo underruns are continuous and the GMCH cannot filter that
interrupt souce.) As we enable our IRQ early so that we can use it
during hardware probing, our interrupt handler must be prepared to
handle a few sources prior to being fully configured. As such, we need
to add a simple is-ready check prior to dereferencing our KMS state for
reporting underruns.

Reported-by: Rob Clark <rclark@redhat.com>
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1193972
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
[Jani: dropped the extra !]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>