drm/i915/gvt: Set ENHANCED_FRAME_CAP bit

Specification says the bit7 of the DPCD MAX_LANE_COUNT (offset 0x02) must
be set to 1 when comes to the displayport version 1.2. This patch respects
the definition.

W/o this patch, guest i915 driver can only set the resolution to 1024*768,
and complains about the unsuccessful link training:

[    5.692193] i915 0000:00:02.0: [drm] *ERROR* index 0, lane_count 1 Link Training Unsuccessful

Fixes: e2e02cbb ("drm/i915/gvt: make dpcd_fix_data supports DP1.2")
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200921065807.247847-1-tina.zhang@intel.com