drm/amd/amdgpu: fix performance drop when VRAM pressure

When VRAM pressue and trigger huge evictions there is performance drop,
this patch fix it.

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