drm/vc4: hdmi: Introduce a scdc_enabled flag

We currently rely on two functions, vc4_hdmi_supports_scrambling() and
vc4_hdmi_mode_needs_scrambling() to determine if we should enable and
disable the scrambler for any given mode.

Since we might need to disable the controller at boot, we also always
run vc4_hdmi_disable_scrambling() and thus call those functions without
a mode yet, which in turns need to make some special casing in order for
it to work.

Instead of duplicating the check for whether or not we need to take care
of the scrambler in both vc4_hdmi_enable_scrambling() and
vc4_hdmi_disable_scrambling(), we can do that check only when we enable
it and store whether or not it's been enabled in our private structure.

We also need to initialize that flag at true to make sure we disable the
scrambler at boot since we can't really know its state yet.

This allows to simplify a bit that part of the driver, and removes one
user of our copy of the CRTC adjusted mode outside of KMS (since
vc4_hdmi_disable_scrambling() might be called from the hotplug interrupt
handler).

It also removes our last user of the legacy encoder->crtc pointer.

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