iris: assert(bo->deps) after realloc()

iris: assert(bo->deps) after realloc()

Iris in general doesn't really like checking the return value of its
allocations, but in some places it does assert that those pointers are
non-NULL. We've recently investigated a bug that could have been
coming from a failed bo->deps realloc(), so add the assert() here to
help give us more confidence over things the next time we're debugging
issues.

Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25236>