drm/amdgpu: Handle null atom context in VBIOS info ioctl

On some APU systems, there is no atom context and so the
atom_context struct is null.

Add a check to the VBIOS_INFO branch of amdgpu_info_ioctl
to handle this case, returning all zeroes.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: David Francis <David.Francis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>