drm/amd/display: Only round InfoFrame refresh rates

[Why]
When calculating nominal refresh rates, don't round.
Only the VSIF needs to be rounded.

[How]
Revert rounding change for nominal and just round when forming the
FreeSync VSIF.

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>