drm/i915/dp: Ensure sink rate values are always valid

Atm, there are no sink rate values set for DP (vs. eDP) sinks until the
DPCD capabilities are successfully read from the sink. During this time
intel_dp->num_common_rates is 0 which can lead to a

intel_dp->common_rates[-1]    (*)

access, which is an undefined behaviour, in the following cases:

- In intel_dp_sync_state(), if the encoder is enabled without a sink
  connected to the encoder's connector (BIOS enabled a monitor, but the
  user unplugged the monitor until the driver loaded).
- In intel_dp_sync_state() if the encoder is enabled with a sink
  connected, but for some reason the DPCD read has failed.
- In intel_dp_compute_link_config() if modesetting a connector without
  a sink connected on it.
- In intel_dp_compute_link_config() if modesetting a connector with a
  a sink connected on it, but before probing the connector first.

To avoid the (*) access in all the above cases, make sure that the sink
rate table - and hence the common rate table - is always valid, by
setting a default minimum sink rate when registering the connector
before anything could use it.

I also considered setting all the DP link rates by default, so that
modesetting with higher resolution modes also succeeds in the last two
cases above. However in case a sink is not connected that would stop
working after the first modeset, due to the LT fallback logic. So this
would need more work, beyond the scope of this fix.

As I mentioned in the previous patch, I don't think the issue this patch
fixes is user visible, however it is an undefined behaviour by
definition and triggers a BUG() in CONFIG_UBSAN builds, hence CC:stable.

v2: Clear the default sink rates, before initializing these for eDP.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/4297
References: https://gitlab.freedesktop.org/drm/intel/-/issues/4298


Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211018143417.1452632-1-imre.deak@intel.com