anv/descriptor_set: Fix binding partly undefined descriptor sets

anv/descriptor_set: Fix binding partly undefined descriptor sets

Section 13.2.3. of the Vulkan spec requires that implementations be able to
bind sparsely-defined Descriptor Sets without any errors or exceptions.

When binding a descriptor set that contains a dynamic buffer binding/descriptor,
the driver attempts to dereference the descriptor's buffer_view field if it is
non-NULL. It currently segfaults on undefined descriptors as this field is never
zero-initialized. Zero undefined descriptors to avoid segfaulting. This
solution was suggested by Jason Ekstrand.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96850
Cc: 12.0 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Nanley Chery <nanley.g.chery@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>