drm/amdgpu: switch the GTT backend to self alloc

Similar to the TTM range manager.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210602100914.46246-6-christian.koenig@amd.com