drm/amdkfd: Don't trigger evictions unmapping dmabuf attachments

Don't move DMABuf attachments for PCIe P2P mappings to the SYSTEM domain
when unmapping. This avoids triggering eviction fences unnecessarily.
Instead do the move to SYSTEM and back to GTT when mapping these
attachments to ensure the SG table gets updated after evictions.

This may still trigger unnecessary evictions if user mode unmaps and
remaps the same BO. However, this is unlikely in real applications.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Eric Huang <jinhuieric.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>