drm/i915: Extract intel_mmio_bar()

We have the same code to determine the MMIO BAR in
two places. Collect it to a single place.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221005154159.18750-1-ville.syrjala@linux.intel.com


Reviewed-by: Matthew Auld <matthew.auld@intel.com>