drm/ttm: ioremap buffer according to TTM mem caching setting

drm/ttm: ioremap buffer according to TTM mem caching setting

If tbo.mem.bus.caching is cached, buffer is intended to be mapped
as cached from CPU. Map it with ioremap_cache.

This wasn't necessary before as device memory was never mapped
as cached from CPU side. It becomes necessary for aldebaran as
device memory is mapped cached from CPU.

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Reviewed-by: Christian Konig <Christian.Koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1614638628-10508-1-git-send-email-Oak.Zeng@amd.com
Signed-off-by: Christian König <christian.koenig@amd.com>