broadcom/compiler: return NULL if we fail to register allocate

broadcom/compiler: return NULL if we fail to register allocate

Right now if we fail to register allocate, we return the qpu_insts
that we had at that point, even if the driver can't really use it.

Also v3dv_pipeline was already assuming that it would return NULL on
failure, returning VK_ERROR_UNKNOWN on that case.

This allows CTS tests with a lot of pressure, that regress now and
then to not being able to allocate, to finish with an error, instead
of blocking forever. For example:
dEQP-VK.graphicsfuzz.spv-stable-maze-flatten-copy-composite

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23203>