drm/i915/gvt: Fix dma-buf display blur issue on CFL

Commit c3b5a843 ("drm/i915/gvt: Enable gfx virtualiztion for CFL")
added the support on CFL. The vgpu emulation hotplug support on CFL was
supposed to be included in that patch. Without the vgpu emulation
hotplug support, the dma-buf based display gives us a blur face.

So fix this issue by adding the vgpu emulation hotplug support on CFL.

Fixes: c3b5a843 ("drm/i915/gvt: Enable gfx virtualiztion for CFL")
Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200227010041.32248-1-tina.zhang@intel.com