drm/amd/display: Fix MST PBN/X.Y value calculations

Changing PBN calculation to be more in line with spec. We don't need to
inflate PBN_NATIVE value by the 1.006 margin, since that is already
taken care of in the get_pbn_per_slot function.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Ilya Bakoulin <ilya.bakoulin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>