drm: Tighten locking in drm_mode_getconnector

drm: Tighten locking in drm_mode_getconnector

- Modeset state needs mode_config->connection mutex, that covers
  figuring out the encoder, and reading properties (since in the
  atomic case those need to look at connector->state).

- Don't hold any locks for stuff that's invariant (i.e. possible
  connectors).

- Same for connector lookup and unref, those don't need any locks.

- And finally the probe stuff is only protected by mode_config->mutex.

While at it updated the kerneldoc for these fields in drm_connector
and add docs explaining what's protected by which locks.

Reviewed-by: Sean Paul <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]