drm/i915/display: PORT_NONE is not valid

Static analysis issue:

In assert_port_valid, add a check to ensure port != PORT_NONE, as that
is not a valid port. The check must be explicit to prevent a bad bit
shift operation in the general case via short-circuiting. It's not
likely this will ever come up in a real use case, but it's at least
worth guarding against.

It would probably also be pertinent to modify the behavior of the
port_name function to correctly print PORT_NONE in this case, as
currently the port would be reported as 'port @' by the debugger. But
that should be done separately, and given port_name is mostly just a
debug printing helper function anyways, fixing it is a low priority.

v2:
- Conditional check was backwards. Fix it. (Jani)

Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patch.msgid.link/20260123152121.7042-2-jonathan.cavitt@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>