On an Asus UX31A (Ivybridge) it is required to load the acpi_video module after the i915kms module to be able to control the brightness (hw.acpi.video.lcd0.brightness) or to successfully resume the machine.
Therefore, it is not possible to directly load the acpi_video module at startup
(especially because loading the i915kms module from loader.conf leads to a black screen on this machine).
Created attachment 201323 [details]
Info of affected system
Text file contains the output of pciconf -lv, /var/run/dmesg.boot, freebsd-version, and uname -a in that order.
System uses a system and kernel built from source, using the default GENERIC configuration for amd64. Had this issue using the default binary updates as well.
New to bugzilla - didn't realize the attaching of files was its own comment.
The system described in the attachment has this same issue, where brightness cannot be controlled if acpi_video is loaded on boot but can be controlled if the module is unloaded and loaded again. The system is a laptop with an Intel Skylake i5 processor and Optimus graphics, using only the Intel card.
I have not run into issues with black screens, though that may be because I'm using the i915kms module from the drm-fbsd12.0-kmod package, loaded using kld_list in rc.conf.