drm/amd/display: Properly round nominal frequency for SPD

[Why]
Some displays rely on the SPD verticle frequency maximum value.
Must round the calculated refresh rate to the nearest integer.

[How]
Round the nominal calculated refresh rate to the nearest whole
integer.

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>