drm/vc4: crtc: Add some logging

The encoder retrieval code has been a source of bugs and glitches in the
past and the crtc <-> encoder association been wrong in a number of
different ways.

Add some logging to quickly spot issues if they occur.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://lore.kernel.org/r/20211025152903.1088803-7-maxime@cerno.tech