drm: Implement simple broken EDID override by loading firmware (v16)

drm: Implement simple broken EDID override by loading firmware (v16)

Main differences between this variant and its gpl analogue:
  * permissive license;
  * no builtin generic EDID data blocks;

Generic EDID firmware can be used from new dports/sysutils/devedid-data port.
Still it is better to use monitor specific EDID block, either by modifying
generic variants or repairing dumped one or even better extracting from
working identical monitor if it is available (look for hex dump in Xorg.0.log).

Make sure that used fw.bin at least has correct resolutions and valid checksum.
This can be done with sysutils/edid-decode utility.

Tested with broken lcd panel returning corrupted EDID block on dvi port:
  * radeonkms R7 240, dvi and dvi2hdmi cable adapter
    echo 'drm.edid_firmware="DVI-D-1:edidfw_repaired"' >> /boot/loader.conf

  * i915 Haswell i7-4712MQ laptop, dvi2hdmi cable adapter
    echo 'drm.edid_firmware="HDMI-A-1:edidfw_1920x1080"' >> /boot/loader.conf
    echo 'drm.video.hdmia1="e"  # force enable on i915' >> /boot/loader.conf

While here, document tunables: drm.video.<connector> and new drm.edid_firmware.