drm/amd/display: Adjust DML workaround threshold

[Why]
There is a case where the margin is between 50 and 60, but applying the
workaround causes a hang. By increasing the threshold, we are blocking more
cases from switching p-state during active, but those cases will fall back
to switching during blank, which is fine.

[How]
 - increase required margin from 50 to 60

Signed-off-by: Joshua Aberback <joshua.aberback@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>