The hardware always writes one or two bytes in the index portion of
an indexed transfer. Make sure the message we send as the index
doesn't have a zero length.

Cc: stable@vger.kernel.org
Cc: Daniel Kurtz <djkurtz@chromium.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sean Paul <seanpaul@chromium.org>
Fixes: 56f9eac0 ("drm/i915/intel_i2c: use INDEX cycles for i2c read transactions")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171123194157.25367-3-ville.syrjala@linux.intel.comReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
(cherry picked from commit bb9e0d4b)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>