graphics/gpu-firmware-kmod: revert "optionize firmware vendors"

This reverts commit 3f78a234e45851fc57d23bd86b6362671244ecc6.

This change was made on assumption that gpu-firmware-kmod is mandatory
for drm users, while in fact drm-kmod which requires gpu-firmware-kmod
is itself an optional metaport. Since the sole purpose of these
metaports is to just install everything, there's no purpose in having
options in them.

Requested by: manu