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.

v2: Pull setting u->_Layer out of the conditional blocks (Jason)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 2208d79d)