panfrost: Fix three buffer overflows

I don't know if the debug option I wrote to catch these deserves to get upstream or not…

Ways to detect other potential buffer overflows:

• Use an ioctl to do the mprotect GPU-side as well, to detect the GPU reading more than was allocated
• Integrate the pool allocator with Valgrind
• Put magic values before and after allocations, and check that they are not overwritten