drm/amdgpu_gem: fix error handling path in amdgpu_gem_va_update_vm

Currently, if amdgpu_vm_bo_update() fails, the returned error
is being ignored.

Fix this by properly checking _r_ after calling amdgpu_vm_bo_update.
Also, remove redundant code just before label _error_.

Addresses-Coverity-ID: 1464280 ("Unused value")
Fixes: 0abc6878 ("drm/amdgpu: update VM PDs after the PTs")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>