drm/i915: Move nv12 chroma plane handling into intel_surf_alignment()

Let's try to keep the alignment requirements in one place, and so
towards that end let's move the AUX_DIST alignment handling into
intel_surf_alignment() alongside the main surface alignment stuff.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170307194210.13400-3-ville.syrjala@linux.intel.com