drm/i915/gvt: Do not allocate initial ring scan buffer

Theoretically, the largest bulk of commands in the ring buffer of an
engine might be the first submission, which usually contains a lot
of commands to initialize the HW. After removing the initial allocation
of the ring scan buffer and let krealloc() do everything we need, we
still have a big chance to get the buffer of suitable size in the first
submission.

Tested on my SKL NUC.

Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>