drm/i915: Check for dev->primary->master before dereference.

I've hit the occasional oops inside i915_wait_ring() with an indication of
a NULL derefence of dev->primary->master.  Adding a NULL check is
consistent with the other potential users of dev->primary->master.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>