drm/i915: Merge sandybridge_pcode_(read|write)

These routines are identical except in the nature of the value parameter.
For writes it is a pure in-param, but for a read, we need an out-param.
Since they differ in a single line, merge the two routines into one.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190426081725.31217-7-chris@chris-wilson.co.uk