drm/xe: add read/write support for MMIO extension space

A distinction has to be made when addressing the MMIO space or
the additional MMIO extension space.

Signed-off-by: Koby Elbaz <kelbaz@habana.ai>
Reviewed-by: Ofir Bitton <obitton@habana.ai>
Reviewed-by: Moti Haimovski <mhaimovski@habana.ai>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>