drm/amd/display: Update DPPCLK programming sequence

[Description]
- When lowering DPPCLK, we want to program the DPP DTO before updating
the DPP refclk.
- Also update DPPCLK to the exact frequency that will be set after clock
divider has been programmed. This will prevent rounding errors when
making the request to PMFW (we need DPP DTO to match exactly with the
exact DPP refclk).

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>