drm/i915: Disable DPLLs before disconnecting the TC PHY

Bspec requires disabling the DPLLs on TC ports before disconnecting the
port's PHY. Add a post_pll_disable encoder hook and move the call to
disconnect the port's PHY from the post_disable hook to the new hook.

Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230323142035.1432621-28-imre.deak@intel.com