Removing viadrm

[coypu%sdf.org@localhost]

Hi folks,

we have two ports of linux drm code. old drm, which exists because not
all devices/drivers work with the newer, also non-x86 architectures use
this.

new drm ("drm2") which hopefully we'll transition to.

there are two via drivers:
viadrmums (drm2)
viadrm (old drm)

according to PR port-i386/53364, viadrm doesn't work any more.
viadrmums does, and uses the newer drm code.

viadrmums shouldn't be significantly different in terms of support (the
driver was almost abandoned upstream until recently).

I don't know of any non-x86 via graphics possible users.

If nobody objects, I will delete viadrm in 1-2 weeks.