drm/i915/intel_i2c: use double-buffered writes

The GMBUS controller GMBUS3 register is double-buffered.  Take advantage
of this  by writing two 4-byte words before the first wait for HW_RDY.
This helps keep the GMBUS controller from becoming idle during long writes.

In fact, during experiments using the GMBUS interrupts, the HW_RDY
interrupt would only trigger for transactions >4 bytes after 2 writes
to GMBUS3.

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>