drm/xe: Drop xe_ prefix from static functions in xe_mmio.c

Rename static functions to align with our typical coding style.
While at it, downgrade the existing kernel-doc for internal
function to normal comment.

Suggested-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240530133527.1328-4-michal.wajdeczko@intel.com