drm/i915/gvt: reset the GGTT entry when vGPU created

The GGTT space is partitioned between vGPUs, it could be reused by
next vGPU after previous one is release, the stale entries need
point to scratch page when vGPU created.

v2: Reset logic move to vGPU create.
v3: Correct the commit msg.
v4: Move the reset function to vGPU init gtt function, as result it's no
    need explicitly in vGPU reset logic as vGPU init gtt called during
    reset.

Signed-off-by: Ping Gao <ping.a.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>