drm/radeon/kms: drop taking lock around crtc lookup.

We only add/remove crtcs at driver load, you cannot remove when
the GPU is running a CS packet since the fd is open, when
GPU hotplugging on radeons actually is needed all this locking
needs a review and I've started re-working kms core locking to deal
with this better. But for now avoid long delays in CS processing when
hotplug detect is happening in a different thread.

this fixes a regression introduced with hotplug detection.

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