drm/xe/pxp: Clear restart flag in pxp_start after jumping back

drm/xe/pxp: Clear restart flag in pxp_start after jumping back

If we don't clear the flag we'll keep jumping back at the beginning of
the function once we reach the end.

Fixes: ccd3c6820a90 ("drm/xe/pxp: Decouple queue addition from PXP start")
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Julia Filipchuk <julia.filipchuk@intel.com>
Reviewed-by: Julia Filipchuk <julia.filipchuk@intel.com>
Link: https://patch.msgid.link/20260324153718.3155504-9-daniele.ceraolospurio@intel.com