xfree86: modesetting: fix warning on unused variable

Fix warning:

../hw/xfree86/drivers/modesetting/driver.c:1612:19: warning: unused variable ‘pEnt’ [-Wunused-variable] 1612 | EntityInfoPtr pEnt = ms->pEnt; | ^~~~

Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net