drm/ttm: add resource iterator v4

Instead of duplicating that at different places add an iterator over all
the resources in a resource manager.

v2: add lockdep annotation and kerneldoc
v3: fix various bugs pointed out by Felix
v4: simplify the code a bit more

Signed-off-by: Christian König <christian.koenig@amd.com>
Tested-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v2)
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220321132601.2161-2-christian.koenig@amd.com