drm/i915/gvt: fix pcode mailbox write emulation of BDW

Add pcode mailbox write emulation in gvt for BDW, reuse emulation code of
Skylake.

V2: refine comments, remove duplication defination of 0x138124, add
IS_SKYLAKE() check for Skylake only pcode commands.

Signed-off-by: Weinan Li <weinan.z.li@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>