drm/i915/skl: Check the DDB state at modeset

v2: Don't check DDB on pre-SKL platforms
    Don't check DDB state on disabled pipes

v3: Squash "Expose skl_ddb_get_hw_state()"

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>