FreeBSD has changed the way graphics drivers are handled on amd64 and i386. Graphics drivers for modern ATI/AMD and Intel graphics cards are now available in ports. The old base drivers are still available and will be installed by default, but they lack support for current generation laptop and desktop systems.
In most cases it is enough to install graphics/drm-kmod from ports or packages to install a driver appropriate for the system, and then by adding the appropriate driver to kld_list in /etc/rc.conf your GPU will be enabled and ready for use.
For Intel (i915) systems after "Broadwell" the /etc/rc.conf entry looks like this: kld_list=/boot/modules/i915kms.ko.
Systems with ATI/AMD graphics cards have two options: Modern systems starting with the HD7000 series GPU should use kld_list=/boot/modules/amdgpu.ko, and for cards released before the HD7000 GPU use kld_list=/boot/modules/radeonkms.ko.
Ensure your user is a member of the "video" group after installing graphics/drm-kmod, failure to do so will result in your user not being able to start Xorg.
When using the graphics drivers from base, or from graphics/drm-legacy-kmod, there are know issues with the xserver driver in x11-drivers/xf86-video-ati, please use xf86-video-ati-legacy instead.
When booting with UEFI and using the ATI/AMD graphics driver, the screen may be garbled before the appropriate driver is loaded.
More information and further details can be found on the FreeBSD graphics team wiki.