drm/i915: Split out i915_priolist_types into its own header

For more intel_engine_mask_t detangling. This time so that we can use
intel_engine_mask_t inside the scheduling structs.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190401162641.10963-2-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>