drm/i915: Disable TRAINING_PATTERN_SET before stopping the TPS transmission

DP spec says:
"The Source device shall start sending the idle pattern after
 it has cleared the Training_Pattern byte in the DPCD."

Currently we do these in operations in the opposite order.
Swap them around to match the spec.

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210118162107.18424-1-ville.syrjala@linux.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>