drm/i915: Don't set up DP ports that aren't in the BIOS device table.

Use the child device array to decide whether the given DP output should be
initialized. If the given DP port can't be found in child device array,
it is not present and won't be initialized.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>