drm/amdgpu: Increase Max GPU instance to 64

Increase Max GPU instances to 64 to handle multi-socket
system with GFX 9.4.3 asic.

Signed-off-by: Mukul Joshi <mukul.joshi@amd.com>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>