drm/xe/pf: Allow to lockdown the PF using custom guard

Some driver components, like eudebug or ccs-mode, can't be used
when VFs are enabled.  Add functions to allow those components
to block the PF from enabling VFs for the requested duration.

Introduce trivial counter to allow lockdown or exclusive access
that can be used in the scenarios where we can't follow the strict
owner semantics as required by the rw_semaphore implementation.

Before enabling VFs, the PF will try to arm the "vfs_enabling"
guard for the exclusive access.  This will fail if there are
some lockdown requests already initiated by the other components.

For testing purposes, add debugfs file which will call these new
functions from the file's open/close hooks.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Christoph Manszewski <christoph.manszewski@intel.com>
Reviewed-by: Christoph Manszewski <christoph.manszewski@intel.com>
Link: https://patch.msgid.link/20251109162451.4779-1-michal.wajdeczko@intel.com

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