drm/i915/gem: Manually dump the debug trace on GEM_BUG_ON

Since igt now defaults to not enabling ftrace-on-oops, we need to
manually invoke GEM_TRACE_DUMP() to see the debug log prior to a
GEM_BUG_ON panicking.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191119100929.2628356-1-chris@chris-wilson.co.uk