drm/i915: Eliminate ironlake_update_primary_plane()

The effective difference between i9xx_update_primary_plane()
and ironlake_update_primary_plane() is only the HSW/BDW
DSPOFFSET special case. So bring that over into
i9xx_update_primary_plane() and eliminate the duplicated code.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170323192712.30682-6-ville.syrjala@linux.intel.com