drm/ttm: add a weak BO reference to the resource v3

Keep track for which BO a resource was allocated.
This is necessary to move the LRU handling into the resources.

A bit problematic is i915 since it tries to use the resource
interface without a BO which is illegal from the conceptional
point of view.

v2: Document that this is a weak reference and add a workaround for i915
v3: further document that this is protected by ttm_device::lru_lock and
    clarify the i915 workaround

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220124122514.1832-4-christian.koenig@amd.com