drm/i915/dp_mst: Fix enabling pipe clock for all streams

commit afb2c443 ("drm/i915/ddi: Push pipe clock enabling to encoders")
inadvertently stopped enabling the pipe clock for any DP-MST stream
after the first one. It also rearranged the pipe clock enabling wrt.
initial MST payload allocation step (which may or may not be a
problem, but it's contrary to the spec.).

Fix things by making the above commit truly a non-functional change.

Fixes: afb2c443

 ("drm/i915/ddi: Push pipe clock enabling to encoders")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107365
Reported-by: Lyude Paul <lyude@redhat.com>
Reported-by:  <dmummenschanz@web.de>
Tested-by:  <dmummenschanz@web.de>
Tested-by: Lyude Paul <lyude@redhat.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: dmummenschanz@web.de
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180831174739.30387-1-imre.deak@intel.com