drm/i915/gt: Remove presumption of RCS0

We now track features correctly instead of probing i915->engine[RCS0]
which is much more flexible and avoids any nasty surprises.

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/20190705124325.14270-2-chris@chris-wilson.co.uk