drm/i915/gvt: Support BAR0 8-byte reads/writes

GGTT is in BAR0 with 8 bytes aligned. With a qemu patch (commit:
38d49e8c1523d97d2191190d3f7b4ce7a0ab5aa3), VFIO can use 8-byte reads/
writes to access it.

This patch is to support the 8-byte GGTT reads/writes.

Ideally, we would like to support 8-byte reads/writes for the total BAR0.
But it needs more work for handling 8-byte MMIO reads/writes.

This patch can fix the issue caused by partial updating GGTT entry, during
guest booting up.

v3:
- Use intel_vgpu_get_bar_gpa() stead. (Zhenyu)
- Include all the GGTT checking logic in gtt_entry(). (Zhenyu)

v2:
- Limit to GGTT entry. (Zhenyu)

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