drm/xe/pf: Add max_vfs configfs attribute to control PF mode

drm/xe/pf: Add max_vfs configfs attribute to control PF mode

In addition to existing max_vfs modparam, add max_vfs configfs
attribute to allow PF configuration on the per-device level.
Default config value is still based on the modparam value.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20251002232648.203370-1-michal.wajdeczko@intel.com