drm/amd/display: Use fixed DET Buffer Size

[why]
Regression from DML1.0 where we use differen DET buffer sizes for each
pipe. From the spec, we need to use DET buffer size of 384 kb for each
pipe

[how]
Ensure to use 384 kb DET buffer sizes for each available pipe.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Sung Joon Kim <sungkim@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>