Resulted in a build failure with -Werror:

../hw/xfree86/drivers/modesetting/drmmode_display.c: In function ‘drmmode_crtc_set_mode’:
../hw/xfree86/drivers/modesetting/drmmode_display.c:759:15: error: unused variable ‘screen’ [-Werror=unused-variable]
  759 |     ScreenPtr screen = crtc->scrn->pScreen;
      |               ^~~~~~

Fixes: c66c548e "modesetting: Call glamor_finish from
                     drmmode_crtc_set_mode"
Reviewed-by: Adam Jackson <ajax@redhat.com>