drm/i915/dp: Preparation for DP phy compliance auto test

During DP phy compliance auto test mode, sink will request
combination of different test pattern with differnt level of
vswing, pre-emphasis. Function added to prepare for it.

Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200316103759.12867-5-animesh.manna@intel.com