st/mesa: fix view template initialization in try_pbo_readpixels

st/mesa: fix view template initialization in try_pbo_readpixels

I think this is what the code was meant to do, albeit as far as I can tell
the redundant initialization some analyzers complain about should work as
well just fine (only the first layer will be used, if the view contains one
or more layers doesn't really matter).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102467
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Cc: mesa-stable@lists.freedesktop.org