drm/i915/mtl: Fix expected reg value for Thunderbolt PLL disabling

While disabling Thunderbolt PLL, we request PLL to be stopped and
wait for ACK bit to be cleared. The expected value should be '0'
instead of '~XELPDP_TBT_CLOCK_ACK' or otherwise we incorrectly
receive dmesg warn "PHY PLL not unlocked in 10us".

Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230512120003.587360-1-mika.kahola@intel.com