Tweak xf86-video-intel for DragonFly 3.7+

* xf86-video-intel 2.21.15+ expects the kernel to set optimal video modes
  on all video outputs

* This is not the case with the current DragonFly implementation. Force
  a reprobe of outputs during X startup

Bug reports:
https://github.com/DragonFlyBSD/DPorts/issues/87
https://bugs.freedesktop.org/show_bug.cgi?id=66494