drm/amdgpu: statically assign gart windows to ttm entities

drm/amdgpu: statically assign gart windows to ttm entities

If multiple entities share the same window we must make sure
that jobs using them are executed sequentially.

This commit gives separate windows to each entity, so jobs
from multiple entities could execute in parallel if needed.
(for now they all use the first sdma engine, so it makes no
difference yet).
The entity stores the gart window offsets to centralize the
"window id" to "window offset" in a single place.

default_entity doesn't get any windows reserved since there is
no use for them.

---
v3:
- renamed gart_window_lock -> lock (Christian)
- added amdgpu_ttm_buffer_entity_init (Christian)
- fixed gart_addr in svm_migrate_gart_map (Felix)
- renamed gart_window_idX -> gart_window_offs[]
- added amdgpu_compute_gart_address
v4:
- u32 -> u64
- added kerneldoc
v5:
- removed gtt_window_lock
- simplified gart window creation and use: entities using a
  single window now uses window #0 instead of #1
- fix dst_addr calculation in kfd_migrate.c
---

Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Acked-by: Felix Kuehling <felix.kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>