drm/i915/intel_i2c: refactor gmbus_xfer

Split out gmbus_xfer_read/write() helper functions.

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>