drm: kerneldoc polish for drm_crtc.c

- Standardized on "Returns:" Block.
- Sprinkle missing kerneldoc over all exported functions and all
  ioctls.
- Add a stern warning that driver's really shouldn't use
  drm_mode_group_init_legacy_group.
- Usual attempt at more consistency.
- Add warnings that drm_mode_object_get/put don't do refcounting,
  despite what the names might lead to believe.
- Try to clarify the framebuffer setup/cleanup functions wrt driver
  private framebuffers - I've fallen recently over this when reviewing
  i915 fbdev patches.
- Align function parameters where the kerneldoc has been updated.
- Most of the drm_get_*_name functions aren't thread safe. Add stern
  warnings where this is the case.

Since a lot of the functions in drm_crtc.c are boilerplate to handle
properties and create default sets of them it might be useful to
extract all that code into a new file drm_property.c. Especially since
properties will be used a lot more in the future.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>