drm/ttm: rename bo->mem and make it a pointer

When we want to decouble resource management from buffer management we need to
be able to handle resources separately.

Add a resource pointer and rename bo->mem so that all code needs to
change to access the pointer instead.

No functional change.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210430092508.60710-4-christian.koenig@amd.com