drm/i915/gvt: Tuning the size of MMIO hash lookup table to 2048

On Skylake platform, The traced virtual mmio registers are up to 2039.
So tuning the hash table size to improve lookup performance.

Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>