drm/i915/icl: program MG_DP_MODE

Programming this register is part of the Enable Sequence for
DisplayPort on ICL. Do as the spec says.

v2: Simple rebase.

Cc: Animesh Manna <animesh.manna@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/20180725002813.6938-5-paulo.r.zanoni@intel.com