drm/amdgpu: fix amdgpu_ttm_bo_eviction_valuable

BOs not mapped into the GART are always valuable for an eviction. Otherwise we
don't correctly swap them out on VRAM evictions during memory pressure.

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>