drm/amd/display: Add guard for FCLK pstate message to PMFW for DCN321

[WHY?]
DCN321 does not support FCLK DPM, and thus it should not send messages to
PMFW regarding it.

Signed-off-by: Dillon Varone <dillon.varone@amd.com>
Acked-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>