drm/xe: Refactor mmio setup for multi-tile

Extract functions to setup the multi-tile mmio space and extension
space, while better documenting the final memory layout.  No change in
behavior.

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240723045558.288693-1-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>