drm/i915: Tidy engine mask types in hangcheck

We can use intel_engine_mask_t to align with the rest of the codebase.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190607082557.31670-2-tvrtko.ursulin@linux.intel.com