amdgpu: Avoid building on UML

The amdgpu driver tries to use fields not supported by UML's cpuinfo
struct. Disable the driver when targeting UML to avoid tripping up
allyesconfig.

e.g.
../drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/smu7_hwmgr.c: In function ‘intel_core_rkl_chk’:
../drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/smu7_hwmgr.c:1742:33: error: initialization of ‘struct cpuinfo_x86 *’ from incompatible pointer type ‘struct cpuinfo_um *’ [-Werror=incompatible-pointer-types
]
../drivers/gpu/drm/amd/amdgpu/../amdkfd/kfd_topology.c: In function ‘kfd_cpumask_to_apic_id’:
../drivers/gpu/drm/amd/amdgpu/../amdkfd/kfd_topology.c:2157:48: error: ‘struct cpuinfo_um’ has no member named ‘apicid’

Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Peter Foley <pefoley2@pefoley.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>