drm/i915: Stop tracking last calculated Sink CRC.

It was created at 'commit aabc95dc (drm/i915: Dont -ETIMEDOUT
on identical new and previous (count, crc).")' becase the counter
wasn't reliable.

Now that we properly wait for the counter to be reset we can rely
a bit more in the counter.

Also that patch stopped to return -ETIMEDOUT so the test case is
unable to skip when it is unreliable and end up in many fails
that should be skip instead.

So, with the counter more reliable we can remove
this hack that just makes things more confusing when test cases
are really expecting the same CRC and let test case skip if that's
not the case.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>