omapdrm: only take crtc->mutex in crtc callbacks

Omapdrm doesn't do anything nefarious with crtc load detection or has
any shared resources, so this is enough. We also need to adjust the
WARN_ON.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Rob Clark <robdclark@gmail.com>