drm/i915: Wait for TypeC PHY complete flag to clear in safe mode

The PHY status complete flag normally clears when disconnecting the PHY
in DP-alt mode (achieved by switching to safe mode), so wait for the
flag to clear.

v2:
- Use DRM_DEBUG_KMS instead of DRM_DEBUG_DRIVER. (José)

Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190628143635.22066-11-imre.deak@intel.com