drm/i915/gvt: add mmio init for virtual display

GVT implements a purely virtual monitor for virtual GPU independent of
the host. Some DDI related MMIO are not initialized in current code
which cause the display initialization failure in guest. This patch
fills the gap.

Signed-off-by: Pei Zhang <pei.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>