drm/i915: Compute the global scheduler caps

Do a pass over all the engines upon starting to determine the global
scheduler capability flags (those that are agreed upon by all).

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/20190226102404.29153-7-chris@chris-wilson.co.uk