drm/i915/dp: Add helpers to set link training mode, BW parameters

Add helpers to set the link mode and BW parameters. These are required
by a follow-up patch setting the parameters for a disabled link.

Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240722165503.2084999-9-imre.deak@intel.com