drm/i915/gvt: Remove defunct vmap_batch()

This code was removed from i915_cmd_parser.c but still an obsolete
version wound up being duplicated into gvt/cmd_parser.c. Good riddance.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>