winsys/amdgpu: sparse buffer creation / destruction / commitment

winsys/amdgpu: sparse buffer creation / destruction / commitment

This is the bulk of the buffer allocation logic. It is fairly simple and
stupid. We'll probably want to use e.g. interval trees at some point to
keep track of commitments, but Mesa doesn't have an implementation of those
yet.

v2:
- remove pipe_mutex_*
- fix total_backing_pages accounting
- simplify by using the new VA_OP_CLEAR/REPLACE kernel interface

Reviewed-by: Marek Olšák <marek.olsak@amd.com>