[WHY]
After unloading driver, driver would not disable DSC function.
At next loading driver, driver would power all DSC engines off.
When driver powered the active DSC off, the screen would be gray
until reprograming DSC relatived register correcntly.

[HOW]
1. Remove DSC Power down code into init_pipes()
2. Depend on  the OTG mapping information and DSC status to skip
power off for the working DSC.
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Yi-Ling Chen <Yi-Ling.Chen2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>