etnaviv: fail in get_handle(TYPE_KMS) without a scanout resource

etnaviv: fail in get_handle(TYPE_KMS) without a scanout resource

The previous logic was returning a handle valid for the render-only
device if rsc->scanout was NULL. However the caller doesn't expect
this: the caller will use the handle with the KMS device.

Instead of returning a handle for the wrong device, fail if we don't
have one.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12074>