drm/xe: Remove dependency on intel_mchbar_regs.h

The only thing really needed is the base offset, MCHBAR_MIRROR_BASE_SNB.
Remove the include and just define it inplace.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>