DragonFlyBSD 3.8 brought Intel DRM support but that only covered the Intel Ivy Bridge graphics hardware and was a port from the Linux 3.8 kernel era. Hitting DragonFlyBSD mainline Git for its kernel is now the Haswell support. While the i915 DRM driver's infrastructure was ported to DragonFly interfaces, adding Haswell support required extra work and still isn't fully operational.
Haswell graphics processors on DragonFlyBSD with this code are reportedly experiencing corruption issues with 2D acceleration and as a result the developers recommend disabling 2D acceleration. More details via this Git commit this morning that landed the Haswell support. This new GPU support should be found in the DragonFlyBSD 3.9 release.