drm/i915: Make sandybridge_pcode_read() deal with the second data register

The pcode mailbox has two data registers. So far we've only ever used
the one, but that's about to change. Expose the second data register to
the callers of sandybridge_pcode_read().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Clint Taylor <Clinton.A.Taylor@intel.com>
Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190521164025.30225-1-ville.syrjala@linux.intel.com