drm/amd/display: reduce sr_xxx_time by 3 us when ppt disable

[Why]
when ppt disabled, the watermark doesn't get fine tune causing
underflow.

[How]
It is a temporary solution to reduce sr_xxx_time by 3 us when ppt
disable.

Signed-off-by: Chiawen Huang <chiawen.huang@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>