drm/i915: Avoid div-by-zero when computing aux_stride w/o an aux plane

To make life easier let's allow skl_plane_stride() to be called for the
AUX surface even when there is no AUX surface. Avoids special cases in
the callers.

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/20170307194210.13400-4-ville.syrjala@linux.intel.com