drm/i915/gvt: changed DDI mode emulation type

changed gvt display transcode DDI mode from DP_SST to
DVI to address below calltrace issue during guest booting
up which is caused by zero dotclock initial value with DP_SST
mode. transcode DVI mode emulation also align with native with DP
connection.

[drm:drm_calc_timestamping_constants]
ERROR crtc 41: Can't calculate constants, dotclock = 0!

WARNING: at drivers/gpu/drm/drm_vblank.c:620
drm_calc_vbltimestamp_from_scanoutpos

Call Trace:
? drm_calc_timestamping_constants+0x144/0x150 [drm]
drm_get_last_vbltimestamp+0x54/0x90 [drm]
drm_reset_vblank_timestamp+0x59/0xd0 [drm]
drm_crtc_vblank_on+0x7b/0xd0 [drm]
intel_modeset_setup_hw_state+0xb67/0xfd0 [i915]
? gen2_read32+0x110/0x110 [i915]
? drm_modeset_lock+0x30/0xa0 [drm]
intel_modeset_init+0x794/0x19d0 [i915]
? intel_setup_gmbus+0x232/0x2e0 [i915]
i915_driver_load+0xb4a/0xf40 [i915]

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