drm/radeon: precompute fence cpu/gpu addr once v3

Add a start fence driver helper function which will be call
once for each ring and will compute cpu/gpu addr for fence
depending on wether to use wb buffer or scratch reg.

This patch replace initialize fence driver separately which
was broken in regard of GPU lockup. The fence list for created,
emited, signaled must be initialize once and only from the
asic init callback not from the startup call back which is
call from the gpu reset.

v2: With this in place we no longer need to know the number of
    rings in fence_driver_init, also writing to the scratch reg
    before knowing its offset is a bad idea.

v3: rebase on top of change to previous patch in the serie

Signed-off-by: Christian König <deathsimple@vodafone.de>
Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>