drm/xe: Add max_vfs module parameter

We want to have an option to limit the number of the VFs that the
PF driver will be able to manage.  With this limit set to zero we
will also have a way to completely disable the PF functionality.

Since we currently don't support SR-IOV on any platform, we start
with this limit set to zero by default.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240404154431.583-2-michal.wajdeczko@intel.com