drm/amd/display: move trace buffer to uncached memory.

[Why & How]
Move dmub trace buffer to uncached memory.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Bindu Ramamurthy <bindu.r@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>