drm/i915/guc: always use Command Transport Buffers

Now that we've moved the Gen9 GuC blobs to version 32 we have CTB
support on all gens, so no need to restrict the usage to Gen11+.
Note that MMIO communication is still required for CTB initialization.

v2: fix commit message nits (Michal)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190606224225.14287-1-daniele.ceraolospurio@intel.com