drm/i915/cnl+: Move the combo PHY init/uninit code to a new file

Similarly to the GEN9_LP DPIO PHY code keep the CNL+ combo PHY code in a
separate file.

No functional change.

v2:
- Use SPDX license tag instead of boilerplate. (Rodrigo)
v3:
- Use MIT instead of GPL-2.0 license. (Ville)

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
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: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181106160621.23057-3-imre.deak@intel.com