drm: extract helpers to compute new training values from sink request

Safe for the minor difference that the intel versions get an offset
into the link_status as an argument, both are the same again.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>