drm/i915/tgl: Limit ourselves to just rcs0

More pruning away of features until we have a stable system and a basis
for debugging what's missing.

v2: Fixup vdbox/vebox fusing

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190913145556.23912-1-chris@chris-wilson.co.uk