drm/amd/display: Fix incorrect rounding for 10Hz refresh range

[Why]
In cases where refresh range is slightly below 10, FreeSync is not
active or supported. Need to round values before checking refresh range
in order to have FreeSync supported in these cases.

[How]
Remove redundant values and round values before checking valid refresh range.

Signed-off-by: Jaehyun Chung <jaehyun.chung@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>