drm/tegra: Refactor CEC support

Most of the CEC support code already lives in the "output" library code.
Move registration and unregistration to the library code as well to make
use of the same code with HDMI on Tegra210 and later via the SOR.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Thierry Reding <treding@nvidia.com>