nv50: make sure to clear _all_ layers of all attachments

nv50: make sure to clear _all_ layers of all attachments

Unfortunately there's only one RT_ARRAY_MODE setting for all
attachments, so clears were previously truncated to the minimum number
of layers any attachment had. Instead set the RT_ARRAY_MODE to 512 (the
max number of layers) before doing the clear. This fixes
gl-3.2-layered-rendering-clear-color-mismatched-layer-count.

Also fix clears of individual layered rt/zeta, in case it ever happens.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Christoph Bumiller <e0425955@student.tuwien.ac.at>
Cc: 10.1 <mesa-stable@lists.freedesktop.org>