drm/amd/display: Correct SSC enable detection for DCN351

drm/amd/display: Correct SSC enable detection for DCN351

[Why]
Due to very small clock register delta between DCN35 and DCN351, clock
spread is being checked on the wrong register for DCN351, causing the
display driver to believe that DPREFCLK downspread to be disabled when
in some stacks it is enabled. This causes the clock values for audio to
be incorrect.

[How]
Both DCN351 and DCN35 use the same clk_mgr, so we modify the DCN35
function that checks for SSC enable to read CLK6 instead of CLK5 when
using DCN351. This allows us to read for DPREFCLK downspread correctly
so the clock can properly compensate when setting values.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Signed-off-by: Kevin Gao <kevin.gao3@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>