drm/xe: Use VF_CAP_REG for device wmb

To force a write barrier on the device memory, we write to the
SOFTWARE_FLAGS_SPR33 register, but this particular register was
selected because it was one of the writable and unused register.

Since a write barrier should also work if we use the read-only
register, switch to VF_CAP_REG register that is also marked as
accessible for VFs.

While at it, add simple kernel-doc for xe_device_wmb() function.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240702183704.1022-4-michal.wajdeczko@intel.com