r600g/compute: Add a function for moving items in the pool

r600g/compute: Add a function for moving items in the pool

This function will be used in the future by compute_memory_defrag
to move items forward in the pool.

It does so by first checking for overlaping ranges, if the ranges
don't overlap it will copy the contents directly. If they overlap
it will try first to make a temporary buffer, if this buffer fails
to allocate, it will finally fall back to a mapping.

Note that it will only be needed to move items forward, it only
checks for overlapping ranges in that case. If needed, it can
easily be added by changing the first if.

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>