drm/i915/gvt: fix a bug of partially write ggtt enties

when guest writes ggtt entries, it could write 8 bytes a time if
gtt_entry_size is 8. But, qemu could split the 8 bytes into 2 consecutive
4-byte writes.

If each 4-byte partial write could trigger a host ggtt write, it is very
possible that a wrong combination is written to the host ggtt. E.g.
the higher 4 bytes is the old value, but the lower 4 bytes is the new
value, and this 8-byte combination is wrong but written to the ggtt, thus
causing bugs.

To handle this condition, we just record the first 4-byte write, then wait
until the second 4-byte write comes and write the combined 64-bit data to
host ggtt table.

To save memory space and to spot partial write as early as possible, we
don't keep this information for every ggtt index. Instread, we just record
the last ggtt write position, and assume the two 4-byte writes come in
consecutively for each vgpu.

This assumption is right based on the characteristic of ggtt entry which
stores memory address. When gtt_entry_size is 8, the guest memory physical
address should be 64 bits, so any sane guest driver should write 8-byte
long data at a time, so 2 consecutive 4-byte writes at the same ggtt index
should be trapped in gvt.

v2:
when incomplete ggtt entry write is located, e.g.
    1. guest only writes 4 bytes at a ggtt offset and no long writes the
       rest 4 bytes.
    2. guest writes 4 bytes of a ggtt offset, then write at other ggtt
       offsets, then return back to write the left 4 bytes of the first
       ggtt offset.
add error handling logic to remap host entry to scratch page, and mark
guest virtual ggtt entry as not present.  (zhenyu wang)

Signed-off-by: Zhao Yan <yan.y.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>