drm/i915/icl: fix icl_unmap/map_plls_to_ports

All connectors may not have best_encoder attached, so don't dereference
encoder pointer for each connector.

Fixes: c27e917e

 ("drm/i915/icl: add basic support for the ICL clocks")
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180525155238.7054-1-lucas.demarchi@intel.com