drm/i915: Check PSR errors instead of retrain while PSR is enabled

When a PSR error happens sink sets the PSR error register and also
set the link status to a error status.
So in the short pulse handling it was returning earlier and doing a
full detection and attempting to retrain but it fails as PSR HW is
in change of the main-link.

Just call intel_psr_short_pulse() before
intel_dp_needs_link_retrain() is not the right fix as
intel_dp_needs_link_retrain() would return true and trigger a full
detection while PSR HW is still in change of main-link.

Check for PSR active is also not safe as it could be inactive due a
frontbuffer invalidate and still doing the PSR exit sequence.

v3: added comment in intel_dp_needs_link_retrain()

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181121225441.18785-2-jose.souza@intel.com