drm/crtc_helper: Replace open-coded CRTC state helpers

Rather than open-coding our own CRTC state helpers, use the atomic helpers
added in f5e7840b, and make our freeing behaviour consistent as well.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Tested-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>