drm/xe: convert PCI barrier mmap to use xe_mmio_gem

Convert the PCI barrier mmap over to use xe_mmio_gem, which is a good
match for this functionality. This has the following advantages:

 1. Removes a bunch of code.
 2. Replaces the fragile hard coded fake offset design.
 3. Adds the first user for xe_mmio_gem, which is preferred over nuking
    it. There are also potentially other upcoming usecases wanting this
    type of functionality, so having standard component to do this would
    be good.

There shouldn't be any big functional change here. From userspace pov,
they still query the fake offset like before, just that now it is no
longer hard coded in the KMD.

v2 (Thomas):
  - Prefer scoped_guard(). Also, just annotate ALL locations, even if
    not strictly needed. Reflect that in the kernel-doc. This will also
    shut up static analysis tools.

Assisted-by: LLM
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Tejas Upadhyay <tejas.upadhyay@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Ilia Levi <ilia.levi@intel.com>
Reviewed-by: Ilia Levi <ilia.levi@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patch.msgid.link/20260908165046.1393557-18-matthew.auld@intel.com

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