drm/i915: Move sandybride pcode access to intel_sideband.c

sandybride_pcode is another sideband, so move it to their new home.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190426081725.31217-8-chris@chris-wilson.co.uk