ilo: try unblocking a transfer with a staging bo

ilo: try unblocking a transfer with a staging bo

When mapping a busy resource with PIPE_TRANSFER_DISCARD_RANGE or
PIPE_TRANSFER_FLUSH_EXPLICIT, we can avoid blocking by allocating and mapping
a staging bo, and emit pipelined copies at proper places.  Since the staging
bo is never bound to GPU, we give it packed layout to save space.