drm/i915: Rename hsw_data_buf_partitioning to intel_ddb_partitioning

We're going to use the 1/2 vs. 5/6 split option already on IVB so the
HSW name is not proper. Just give it an intel_ prefix and move it to
i915_drv.h so that we can use it there later.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>