drm/i915/gvt: Enable virtual display support for BXT.

Virtual monitor on BXT start from port B.
Unlike SKL/KBL, digital display port connectivity is detected via
GEN8_DE_PORT_ISR so emulate monitor state change by setting it.

Signed-off-by: Colin Xu <colin.xu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>