drm/radeon: rework the VM code a bit more (v2)

Roughly based on how nouveau is handling it. Instead of
adding the bo_va when the address is set add the bo_va
when the handle is opened, but set the address to zero
until userspace tells us where to place it.

This fixes another bunch of problems with glamor.

v2: agd5f: fix build after dropping patch 7/8.

Signed-off-by: Christian König <deathsimple@vodafone.de>