drm/vc4: crtc: Fix vc4_get_crtc_encoder logic

The vc4_get_crtc_encoder function currently only works when the
connector->state->crtc pointer is set, which is only true when the
connector is currently enabled.

However, we use it as part of the disable path as well, and our lookup
will fail in that case, resulting in it returning a null pointer we
can't act on.

We can access the connector that used to be connected to that crtc
though using the old connector state in the disable path.

Since we want to support both the enable and disable path, we can
support it by passing the state accessor variant as a function pointer,
together with the atomic state.

Fixes: 792c3132 ("drm/vc4: encoder: Add finer-grained encoder callbacks")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210507150515.257424-5-maxime@cerno.tech