drm/xe/mmio: Move xe_mmio_wait32() to xe_mmio.c

drm/xe/mmio: Move xe_mmio_wait32() to xe_mmio.c

This function is big enough, let's move it to a shared compilation unit.

While at it, document it.

Here is the output of running bloat-o-metter on the new and old module
(execution provided by Lucas):

    $ ./scripts/bloat-o-meter build64/drivers/gpu/drm/xe/xe.ko{.old,}
    add/remove: 2/0 grow/shrink: 0/58 up/down: 554/-15645 (-15091)
    (...) # Lines in between omitted
    Total: Before=2181322, After=2166231, chg -0.69%

The overall reduction in the size is not that significant. Nevertheless,
keeping the function as inline arguably does not bring too much benefit
as well.

As noted by Lucas, we would probably benefit from an inline
function that did the fast-path check: do an optimistic first check
before entering the wait-logic, which itself would go to a compilation
unit. We might come back to implement this in the future if we have data
to justify it.

v2:
  - Add note in documentation for @timeout_us regarding the exponential
    backoff strategy. (Lucas)
  - Share output of bloat-o-meter in the commit message. (Lucas)

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20231116214000.70573-2-gustavo.sousa@intel.com
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>