drm/i915: Replace the feature tests for BLT/BSD with ring init checks

When userspace asks whether the driver supports the BLT or BSD rings for
this chip, simply report whether those particular rings are initialised

v2: Use intel_ring_initialized()

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>