drm/radeon/kms: move modeset init outside of GPU init

We are splitting GPU & modeset init so that it's easier
to abord only remaining GPU init when somethings fails.
We want to always provide enough funcionalities to get
fbcon and a shadowfb X working. Only acceptable error
during initialization are memory allocation failure or
io mapping failure.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>