drm/i915: Force sink crc stop before start.

By Vesa DP spec, test counter at DP_TEST_SINK_MISC just reset to 0
when unsetting DP_TEST_SINK_START, so let's force this stop here.

But let's minimize the aux transactions and just do it when we know
it hasn't been properly stoped.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>