drm/i915/gvt: Remove duplicated MMIO entries

Remove duplicated MMIO entries in the tracked MMIO list. -EEXIST
is returned if duplicated MMIO entries are found when new MMIO
entry is added.

v2:
- Use WARN(1, ...) for more verbose message. (Zhenyu)

Signed-off-by: Jian Jun Chen <jian.jun.chen@intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Cc: Changbin Du <changbin.du@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Reviewed-by: Yulei Zhang <yulei.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>