drm/i915: Just check the vebox IIR regardless

As we don't unmask and enable the vebox interrupts if the engine is not
being used, we will never generate the vebox interrupts as part of the
IIR and so can unconditionally check IIR without fear of chasing into
the vebox.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190305150914.11340-1-chris@chris-wilson.co.uk