drm/i915/gvt: Rename reserved ring buffer

"reserved" means reserve something from somewhere. Actually they are
buffers used by command scanner. Rename it to ring_scan_buffer.

v2:

- Remove the usage of an extra variable. (Zhenyu)

Fixes: 0a53bc07 ("drm/i915/gvt: Separate cmd scan from request allocation")
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>