drm/amd/display: WBSCL filter init calculation fixes

* Previous code did some calculations with a mix of normal integers and
  integers aligned as U2.24 fixed-point values.
* There were bugs in the conversion of the final result into the
  S4.19 values required for the registers.

Signed-off-by: Ken Chalmers <ken.chalmers@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>