drm/amdgpu: stop mapping BOs to GTT

No need to map BOs to GTT on eviction and intermediate transfers any more.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>