drm/amd/display: Increase minimum clock for TMDS 420 with pipe splitting

drm/amd/display: Increase minimum clock for TMDS 420 with pipe splitting

[Why]
-Pipe splitting allows for clocks to be reduced, but when using TMDS 420,
reduced clocks lead to missed clocks cycles on clock resyncing

[How]
-Impose a minimum clock when using TMDS 420

Reviewed-by: Chris Park <chris.park@amd.com>
Signed-off-by: Relja Vojvodic <rvojvodi@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>