Backporting this TH05 innovation to TH01. Dynamic far pointer
construction from the SEG_PLANE_* constants might be faster, but
definitely not smaller.
Also raises the question of how we organize the files during the long
time in which TH02-TH04 are not position-independent and thus still
need vram_planes_set() for building into identical binaries. Moving the
function definition up to TH02 is *kinda* unauthentic, but this branch
ultimately doesn't care.
Part of P0229, funded by Ember2528.
nmlgc