The only relevant difference between i9xx_get_initial_plane_config() and
ironlake_get_initial_plane_config() is the HSW/BDW TILEOFF handling.
Add that to i9xx_get_initial_plane_config() and nuke
ironlake_get_initial_plane_config().

v2: s/plane/i9xx_plane/ etc. (James)

Cc: James Ausmus <james.ausmus@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171117191917.11506-7-ville.syrjala@linux.intel.comSigned-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>