Pull the code to disable the port clock into a function. We already have
the intel_ddi_clk_select() counterpart.

v2: Keep using intel_ddi_get_encoder_port() for now (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171010121207.570-3-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>