drm: remove agp_init() bus callback

The PCI bus helper is the only user of it. Call it directly before
device-registration to get rid of the callback.

Note that all drm_agp_*() calls are locked with the drm-global-mutex so we
need to explicitly lock it during initialization. It's not really clear
why it's needed, but lets be safe.

v2: Rebase on top of the agp_init interface change.

v3: Remove the rebase-fail where I've accidentally killed the ->irq_by_busid
callback a bit too early.

Cc: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>