When I include:

amdgpu_load="YES"
amdgpu_name="/boot/modules/amdgpu.ko

in my /boot/loader.conf, it loads amdgpu.ko, and also loads drm.ko, linuxkpi_gplv2.ko, dmabuf.ko, and ttm.ko (which I hypothesize happens because the top Makefile specifies:

DEFAULT_KMODS=  dmabuf          \
                linuxkpi        \
                ttm             \
                drm             \
                amd             \
                radeon

But in order to work, we also need firmware for my graphics card to load, which means amdgpu_raven_{gpu_info,sdma,asd,ta,pfp,me,ce,rlc,mec,mec2,vcn}_bin.ko also have to be loaded in my case.  And they are, if I kldload amdgpu.ko by hand in single-user mode or include amdgpu.ko in kld_list in my /etc/rc.conf.  (And in that case, X windows comes up nicely using the "amdgpu" driver, whereas otherwise it comes up in VESA mode.)

In conjunction with bug #267028, it would be nice if I could load amdgpu.ko at /boot/loader.conf time, since loading it later on increases the chance of a kernel crash in ZFS code.  Don't ask me why.