[why]
The mux to switch between refclk and dto_dsc_clk is non double buffered.
However dto dsc clk's phase and modulo divider registers are currently
configured as double buffered update. This causes a problem when we switch to
use dto dsc clk and program phase and modulo in the same sequence. In this
sequence dsc clk is switched to dto but the clock divider programming doesn't
take effect until next frame. When we try to program DSCC registers, SMN bus
will hang because dto dsc clk divider phase is set to 0.

[how]
Configure phase and modulo to take effect immediately. Always switch to dto dsc
clk before DSC clock is unagted. Switch back to refclk after DSC clock is gated.
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Reviewed-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>