drm/i915/display/tgl: Fix the order of the step to turn transcoder clock off

For TGL the step to turn off the transcoder clock was moved to after
the complete shutdown of DDI. Only the MST slave transcoders should
disable the clock before that.

v2:
- Adding last_mst_stream to intel_mst_post_disable_dp, make code more
easy to read and is similar to first_mst_stream in
intel_mst_pre_enable_dp()(Ville's idea)
- Calling intel_ddi_disable_pipe_clock() for GEN12+ right
intel_disable_ddi_buf() as stated in BSpec(Ville)

BSpec: 49190
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191205210350.96795-2-jose.souza@intel.com