drm/xe: Initialize ret in mcr_lock()

ret is not initialized in mcr_lock() when running in platforms with
graphics IP version < 1270, this could cause drm_WARN_ON_ONCE()
to hit eventually(what just happened to me).

Fixes: dd08ebf6 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>