drm/amd/display: Calculate link bandwidth in a common function

[why]
Currently link bandwidth is calculated in two places, using the same
formula. They should be unified into calling one function.

[how]
Replace all implementations of link bandwidth calculation with a call
to a function.

Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Nikola Cornij <Nikola.Cornij@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>