drm/vc4: hdmi: Reset link on hotplug

During a hotplug cycle (such as a TV going out of suspend, or when the
cable is disconnected and reconnected), the expectation is that the same
state used before the disconnection is reused until the next commit.

However, the HDMI scrambling requires that some flags are set in the
monitor, and those flags are very likely to be reset when the cable has
been disconnected. This will thus result in a blank display, even if the
display pipeline configuration hasn't been modified or is in the exact
same state.

The solution we've had so far is to enable the scrambling-related bits
again on reconnection, but the HDMI 2.0 specification (Section 6.1.3.1 -
Scrambling Control) requires that the scrambling enable bit is set
before sending any scrambled video signal. Using that solution thus
breaks that expectation.

The solution used by i915 is to do a full modeset on the connector so
that we disable the video signal, enable the scrambling bit, and enable
the video signal again.

As such, we took that code and plugged it into vc4. It probably could
have been turned into an helper, but it proved to be difficult for
several reasons:

  * i915 has fairly different structures than simpler KMS drivers such
    as vc4, so doing some code that works with both proved to be
    difficult;

  * Other simpler drivers could reuse some of it (tegra, dw-hdmi), but
    it would still require to move some parameters currently stored in
    private structure that are needed to compute whether the scrambling
    is needed or not, and then inform the driver that it needs to be
    enabled. Some of those parameters are already in core structures
    (drm_display_mode, drm_display_info, bpc), but the output format
    isnt't. Adding it is fairly challenging since unlike the TMDS char
    rate or mode, there's no consensus on what format to pick in
    drivers, so it's not possible to write some generic code that can
    depend on it.

For these reasons, we chose to duplicate the code for now, until someone
else really needs it as well, in which case we will be able to convert
it into a generic helper.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220829134731.213478-8-maxime@cerno.tech