drm/xe: Add helper to calculate adjusted register offset

Our MMIO accessing functions automatically adjust addresses for the
media registers based on mmio.adj_limit and mmio.adj_offset logic.
Move it to the separate helper to avoid code duplication and to
allow using it by the upcoming changes to PF driver code.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Piotr Piórkowski <piotr.piorkowski@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240423180436.2089-3-michal.wajdeczko@intel.com