drm/i915/gt: debugfs: Evaluate forcewake usage within locks

The forcewake count and domains listing is multi process critical
and the uncore provides a spinlock for such cases.

Lock the forcewake evaluation section in the fw_domains_show()
debugfs interface.

Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240607145131.217251-1-andi.shyti@linux.intel.com