drm/amd/display: Lowering min Z8 residency time

[Why & How]
Per HW team request, we're lowering the minimum Z8
residency time to 2000us. This enables Z8 support for additional
modes we were previously blocking like 2k>60hz

Cc: stable@vger.kernel.org
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Leo Chen <sancchen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>