drm/xe/xe2: Make subsequent L2 flush sequential

drm/xe/xe2: Make subsequent L2 flush sequential

Issuing the flush on top of an ongoing flush is not desirable.
Lets use lock to make it sequential.

Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Signed-off-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240710052750.3031586-1-tejas.upadhyay@intel.com
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
(cherry picked from commit 71733b8d7f50b61403f940c6c9745fb3a9b98dcb)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>