Assembler/bdw: Remove the unsupported cache agent for WRITE(...)

The Sampler/Constant cache is read-only. And it can't be used as
the target cache agent of WRITE message.

Reviewed-by: Xiang, Haihao <haihao.xiang@intel.com>
Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>