mesa: don't forget to clear _Layer field on texture unit

On the Android Antutu benchmark we ran into an assert in ISL where the (base layer + num layers) > total layers. It turns out the core of mesa forgot to clear the _Layer variable, potentially leaving an inconsistent value.

Signed-off-by: Lionel Landwerlin lionel.g.landwerlin@intel.com Cc: mesa-stable@lists.freedesktop.org