Revert "drm/vc4: Rework the encoder retrieval code"

This reverts commit 3cf3d39b.

This commit was making the assumption that we had a 1:1 mapping between
the encoders and their CRTC. While this is true for the HDMI controllers
on the BCM2711, this isn't true for the other encoders (DSI0 and DPI
share the PixelValve 0, and DSI1 and SMI share the PixelValve1), and
this isn't true at all on the older SoCs, effectively breaking the
encoder retrieval logic.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>