drm/i915: Fix primary plane offset on HSW

Haswell consolidates DSP_TILEOFF and DSP_LINOFF into DSP_OFFSET (aka
PRI_OFFSET).

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>