drm/i915: Retry DP aux_ch communications with a different clock after failure

The w/a db makes the recommendation to both use a non-default value for
the initial clock and then to retry with an alternative clock for
Haswell with the Lakeport PCH.

"On LPT:H, use a divider value of 63 decimal (03Fh). If there is a
failure, retry at least three times with 63, then retry at least three
times with 72 decimal (048h)."

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>