drm/i915/icl: toggle PHY clock gating around link training

The Gen11 TypeC PHY DDI Buffer chapter, PHY Clock Gating Programming
section says that PHY clock gating should be disabled before starting
voltage swing programming, then enabled after any link training is
complete.

v2: Simple rebase.

Cc: Animesh Manna <animesh.manna@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> (v1)
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180725002813.6938-6-paulo.r.zanoni@intel.com