drm/i915/gvt: Remove one duplicated MMIO

Remove one duplicated MMIO GEN6_PCODE_MAILBOX. Duplicated MMIO will
cause host GVT-g initialization failure.

Fixes: 9c3a16c8 ("drm/i915/hsw+: Add support for multiple power well regs")
Signed-off-by: Jian Jun Chen <jian.jun.chen@intel.com>
Acked-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>