drm/i915: Flush using only the correct base address register

We were writing DSP_ADDR and DSP_SURF unconditionally. This did not
trigger an unclaimed write before HSW as the address of DSP_ADDR has
been repurposed as DSP_LINOFF.

On HSW, though, DSP_LINOFF has been removed and then writting to it
triggers an unclaimed write.

This patch writes to DSP_ADDR or DSP_SURF to flush the display plane
configuration depending on the gen we're running on.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>