drm/i915/gvt: Fix mmio handler break on BXT/APL.

- Remove dup mmio handler for BXT/APL. Otherwise mmio handler will fail
  to init.
- Add engine GPR with F_CMD_ACCESS since BXT/APL will load them via
  LRI. Otherwise, guest will enter failsafe mode.

V2:
Use RCS/BCS GPR macros instead of offset.
Revise commit message.

V3:
Use GEN8_RING_CS_GPR macros on ring base.

Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Colin Xu <colin.xu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20201016052913.209248-1-colin.xu@intel.com