drm/amd/amdgpu: Add missing "DDR4" label

The commit d296278fd372003fc69588acfd0c0c5edbdf4874 added support for
detecting DDR4 but omitted the label that is printed out in
amdgpu_bo_init() resulting in a KASAN error.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>