drm/amd/display: Force prefetch mode to 0

[Why]
On APU should be always using prefetch mode 0.
Currently, sometimes prefetch mode 1 is being
used causing system to hard hang due to
minTTUVBlank being too low.

[How]
Any ASIC running DCN21 will by default allow
self refresh and mclk switch. This sets both
min and max prefetch mode to 0 by default.

Signed-off-by: Isabel Zhang <isabel.zhang@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>