drm/i915/gvt: Fix shift for cmd data size

This is to fix smatch warning on

     drivers/gpu/drm/i915/gvt/cmd_parser.c:1421 cmd_handler_mi_op_2f()
     warn: shift has higher precedence than mask

We need bits 20-19 mask for data size.

Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>