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>

drivers/gpu/drm/amd/amdgpu/amdgpu_device.c

@@ -1073,6 +1073,10 @@ def_value:
 
 static void amdgpu_check_vm_size(struct amdgpu_device *adev)
 {
+	/* no need to check the default value */
+	if (amdgpu_vm_size == -1)
+		return;
+
 	if (!amdgpu_check_pot_argument(amdgpu_vm_size)) {
 		dev_warn(adev->dev, "VM size (%d) must be a power of 2\n",
 			 amdgpu_vm_size);