drm/ttm: Remove unncessary retval from ttm_bo_vm_fault()

The dual ret/retval was more complex than need be.  Now
we drop the retval variable and assign the appropriate VM
codes to ret instead.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>