drm/i915/dp: Send DPCD ON for MST before phy_up

drm/i915/dp: Send DPCD ON for MST before phy_up

When doing a modeset where the sink is transitioning from D3 to D0 , it
would sometimes be possible for the initial power_up_phy() to start
timing out. This would only be observed in the last action before the
sink went into D3 mode was intel_dp_sink_dpms(DRM_MODE_DPMS_OFF). We
originally thought this might be an issue with us accidentally shutting
off the aux block when putting the sink into D3, but since the DP spec
mandates that sinks must wake up within 1ms while we have 100ms to
respond to an ESI irq, this didn't really add up. Turns out that the
problem is more subtle then that:

It turns out that the timeout is from us not enabling DPMS on the MST
hub before actually trying to initiate sideband communications. This
would cause the first sideband communication (power_up_phy()), to start
timing out because the sink wasn't ready to respond. Afterwards, we
would call intel_dp_sink_dpms(DRM_MODE_DPMS_ON) in
intel_ddi_pre_enable_dp(), which would actually result in waking up the
sink so that sideband requests would work again.

Since DPMS is what lets us actually bring the hub up into a state where
sideband communications become functional again, we just need to make
sure to enable DPMS on the display before attempting to perform sideband
communications.

Changes since v1:
- Remove comment above if (!intel_dp->is_mst) - vsryjala
- Move intel_dp_sink_dpms() for MST into intel_dp_post_disable_mst() to
  keep enable/disable paths symmetrical
- Improve commit message - dhnkrn
Changes since v2:
- Only send DPMS off when we're disabling the last sink, and only send
  DPMS on when we're enabling the first sink - dhnkrn
Changes since v3:
- Check against is_mst, not intel_dp->is_mst - dhnkrn/vsyrjala

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Laura Abbott <labbott@redhat.com>
Cc: stable@vger.kernel.org
Fixes: ad260ab32a4d9 ("drm/i915/dp: Write to SET_POWER dpcd to enable MST hub.")
Link: https://patchwork.freedesktop.org/patch/msgid/20180407011053.22437-1-lyude@redhat.com