drm: remove global_mutex locking around agp_init

David Herrmann dutifully moved this locking along when moving the
agp_init call out of the generic drm_dev_register into the pci
specific load helpers.

But afaict there's no need and the reason for that locking has been
purely a historical accident - we need the lock around the driver dev
node registration to paper over the midlayer init races, and the agp
init simply ended up in there. The real fix for all this is of course
to delay the dev (and sysfs/debugfs) interface registration until
everything is fully set up.

Until then stop the cargo-cult locking from spreading and remove the
locking.

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