drm/amdgpu: return -EFAULT if copy_to_user() fails

The copy_to_user() function returns the number of bytes remaining to be
copied, but we want to return a negative error code to the user.

Fixes: 030d5b97 ("drm/amdgpu: use amdgpu_device_vram_access in amdgpu_ttm_vram_read")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>