drm/i915: Do not rely on for loop caching the mask

for_each_engine_masked caches the engine mask but what does the caller
know.

Cache it explicitly for clarity and while at it correct the type to match.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190725125056.11942-1-tvrtko.ursulin@linux.intel.com