drm/i915: Sink CRC: tune down error message at stop to debug_kms.

When we stop the sink CRC calculation we wait a while until the counter
is reset to zero and return -ETIMEDOUT. However the sink crc was
calculated already by this point so we just ignore this return at
the main function.

So, let's also ignore the message and put it as a debug message instead
of an error one. The message might still be useful when debuging
test failures so we could be able to know something was not going so
well with sink crc stop.

v2: Improve log message.

Reference: https://bugs.freedesktop.org/show_bug.cgi?id=93694


Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1454107499-29678-1-git-send-email-rodrigo.vivi@intel.com