drm/i915/dp_mst: Add a way to disable the DP2 config

drm/i915/dp_mst: Add a way to disable the DP2 config

Add a way to disable the DP2 configuration, required by the next patch
during transcoder disabling.

While at it drop the redundant encoder parameter.

v2:
- Keep intel_ddi_config_transcoder_dp2() static. (Jani)
- Remove the encoder parameter.

Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241030192313.4030617-4-imre.deak@intel.com