drm/radeon/kms: resume AGP by calling init.

AGP resume was broken since we moved to the new init path,
because we never re-enabled AGP on these systems at resume time.

This patch just calls the AGP resume call which just does the reinit
at resume time like the old path did.

Since AGP is pretty much gpu independant I did it outside
the gpu specific code.

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