drm/crtc: Polish kerneldoc

- Make it clear that it's a negative errno (more in line with
  everything else).
- Clean up the confusion around get_properties vs. getproperty ioctls:
  One reads per-obj property values, the other reads property
  metadata.
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>