drm/amdgpu: Fix error codes if copy_to_user() fails

The copy_to_user() function returns the number of bytes that it wasn't
able to copy, but we should return -EFAULT to the user.

Fixes: 4d82724f ("drm/amdgpu: Add mapping info option for GEM_OP ioctl")
Fixes: f9db1fc5 ("drm/amdgpu: Add ioctl to get all gem handles for a process")
Reviewed-By: David Francis <David.Francis@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>