drm/i915/intel_i2c: always wait for IDLE before clearing NAK

The GMBUS controller can report a NAK condition while a transaction is
still active. If the driver is fast enough, and the bus is slow enough,
the driver may clear the NAK condition while the controller is still
busy, resulting in a confused GMBUS controller.  This will leave the
controller in a bad state such that the next transaction may fail.

Also, return -ENXIO if a device NAKs a transaction.

Note: this patch also refactors gmbus_xfer to remove the "done" label.

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>