amdgpu: fix a valgrind warning

==10307== Conditional jump or move depends on uninitialised value(s)
==10307==    at 0xB9F4CFA: amdgpu_device_initialize (amdgpu_device.c:193)

Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>