drm/amd/display: move bw calc code into helpers

[Why]
For better readability and reusability

[How]
Move snippets of BW calculation code into helpers.

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Fatemeh Darbehani <Fatemeh.Darbehani@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>