Factor out a function verifying the PHY connected state in legacy or
DP-alt mode. This is common to all platforms, which can be reused in
platform specific connect hooks added in follow-up patches.

No functional changes.
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230323142035.1432621-13-imre.deak@intel.com