drm/i915: Make intel_engine_dump irqsafe

To be useful later, enable intel_engine_dump() to be called from irq
context (i.e. using saving and restoring irq start rather than assuming
we enter with irqs enabled).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180518090212.5349-1-chris@chris-wilson.co.uk