drm/amdgpu: evict vram when gpu reset

On workstation cards with ECC vram, the entirety of vram is cleared to 0
on asic init to set the ECC status correctly.  On non ECC boards, I don't
think they do any explicit clearing, but the vram controller is reset
which may cause issues with the data there.

Signed-off-by: Chunming Zhou <David1.Zhou@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>