drm/vc4: Add CTM registers to debugfs

Now that we set the OLED* registers to do CTM, it's helpful to have them
in the register dump.

Signed-off-by: Stefan Schake <stschake@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180420122545.40014-2-stschake@gmail.com