drm/i915: don't write PLANE_BUF_CFG twice every time

We were writing to PLANE_BUF_CFG(pipe, plane_id) twice for every
platform, and we were even using different values on the gen10- planar
case. The first write is useless since it just gets replaced with the
next one, so kill it.

There's a lot to improve in the DDB code, but let's start by avoiding
the double write.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004231600.14101-6-paulo.r.zanoni@intel.com