drm/i915: fix error path in intel_setup_gmbus()

This fails to undo the setup for pin==0; moreover, something
interesting happens if the setup failed already at pin==0.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Fixes: f899fc64

 ("drm/i915: use GMBUS to manage i2c links")
Cc: stable@vger.kernel.org
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455048677-19882-3-git-send-email-linux@rasmusvillemoes.dk
(cherry picked from commit 2417c8c0

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>