nvc0: we have 16k-sized framebuffers, fix default scissors

For some reason we don't use view volume clipping by default, and use
scissors instead. These scissors were set to an 8k max fb size, while
the driver advertises 16k-sized framebuffers.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: <mesa-stable@lists.freedesktop.org>
(cherry picked from commit cc79a148)