drm/i915/icl: Sanitize DDI port clock gating for DSI ports

The requirement for the DDI port clock gating for a port in DSI mode is
the opposite wrt. the case when the port is in DDI mode: the clock
should be gated when the port is active and ungated when the port is
inactive. Note that we cannot simply keep the DDI clock gated when the
port will be only used in DSI mode: it must be gated/ungated at a
specific spot in the DSI enable/disable sequence.

Ensure the above for all ports of a DSI encoder, also adding a sanity
check that we haven't registered another encoder using the same port
(VBT should never allow this to happen).

Cc: Madhav Chauhan <madhav.chauhan@intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Clint Taylor <clinton.a.taylor@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ceb14d5a68e8e23988d923d4290a4f981789e616.1543500285.git.jani.nikula@intel.com