drm/amdgpu: don't check the default value for vm size

Avoids printing spurious messages like this:
[    3.102059] amdgpu 0000:01:00.0: VM size (-1) must be a power of 2

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>