drm/amd/display: Add check for forced_clocks debug option

drm/amd/display: Add check for forced_clocks debug option

[why]
driver missed the check.

[how]
add the check.
add min display clock = 100mhz check based on dccg doc.

[note]
add SetPhyclkVoltageByFreq as confirmed with smu, but not enabled in
this change.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>