drm/i915/dp: Write to SET_POWER dpcd to enable MST hub.

If bios sets up an MST output and hardware state readout code sees this is
an SST configuration, when disabling the encoder we end up calling
->post_disable_dp() hook instead of the MST version. Consequently, we write
to the DP_SET_POWER dpcd to set it D3 state. Further along when we try
enable the encoder in MST mode, POWER_UP_PHY transaction fails to power up
the MST hub. This results in continuous link training failures which keep
the system busy delaying boot. We could identify bios MST boot discrepancy
and handle it accordingly but a simple way to solve this is to write to the
DP_SET_POWER dpcd for MST too.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105470


Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reported-by: Laura Abbott <labbott@redhat.com>
Cc: stable@vger.kernel.org
Fixes: 5ea2355a

 ("drm/i915/mst: Use MST sideband message transactions for dpms control")
Tested-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180314054825.1718-1-dhinakaran.pandiyan@intel.com
(cherry picked from commit ad260ab3

)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>