drm/amdgpu: move validation of the VM size into the VM code

This moves validation of the VM size parameter into amdgpu_vm_adjust_size().

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>