drm/i915: Assert that the port being initialized is valid

Sprinkle some asserts to catch any mishaps in the port_mask
vs. output init.

For DDI/DP/HDMI/SDVO I decided that we want to bail out for
an invalid port since those are the encoder types where
we might want consider driving the whole thing from the VBT
child device list, and bogus VBTs could be a real issue
(if for no other reason than the i915.vbt_firmware).

For DVO and HSW/BDW CRT port I just threw the assert in
there for good measure.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230616140820.11726-5-ville.syrjala@linux.intel.com