UPSTREAM: drm/i915/intel_i2c: handle zero-length writes

UPSTREAM: drm/i915/intel_i2c: handle zero-length writes

A common method of probing an i2c bus is trying to do a zero-length write.
Handle this case by checking the length first before decrementing it.

This is actually important, since attempting a zero-length write is one
of the ways that i2cdetect and i2c_new_probed_device detect whether
there is device present on the bus with a given address.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
(cherry picked from commit 26883c31b0799e76edf8f0ea8be48b64e09b2a7d)

Change-Id: I92da830afe16684d9d3e9f2cf80a667d874ee602
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>