drm/i915: add single combo phy init/unit functions

Work on the principle that files should prefer not to expose platform
specific functions.

v2, v3: Rebase

Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190502145234.7002-1-jani.nikula@intel.com