drm/i915: Iterate all child devs in intel_bios_is_port_present()

drm/i915: Iterate all child devs in intel_bios_is_port_present()

Instead of consulting vbt.ports[] lets just go through the
whole child device list to check whether a specific port
was declared by the VBT or not.

Note that this doesn't change anything wrt. detecting duplicate
child devices with the same port as vbt.ports[] would also always
contain exactly one of the duplicates.

v2: Include a is_port_valid() check to deal with some broken VBTs
    Mention something about duplicate port detection (Jani)

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230214073818.20231-1-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>