drm/amd/display: Do 1-to-1 mapping between OPP and DSC in DML2

[why]
To determine which block instance to power-gate,
we look at the available pipe resource for both plane
and stream. On MPO, DSC3 was falsely powered on even
though only 1 stream path was enabled because
the resource mapping was not done correctly.

[how]
Acquire the correct DSC instance to power on / off based
on the instance of OPP which determines the backend
pipe index.

Reviewed-by: Swapnil Patel <swapnil.patel@amd.com>
Signed-off-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Sung Joon Kim <sungjoon.kim@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>