drm/ttm: allocate resource object instead of embedding it v2

To improve the handling we want the establish the resource object as base
class for the backend allocations.

v2: add missing error handling

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210602100914.46246-1-christian.koenig@amd.com