drm/i915/gvt: Fix display port type issue

According to the vbt provided by GVT-g, the display output type should
be display port, instead of DVI. Currently, GVT-g display model only
supports display port emulation, not DVI or HDMI. This patch fixes this
by telling guest i915 the right output type supported by GVT-g.

v3: make port_b and port_c use dp type connector too. (Zhenyu)
v2: refine the commit message. (Zhenyu)

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/20200317093615.10538-1-tina.zhang@intel.com