drm/amdgpu: use the TTM dummy page instead of allocating one

We have a global dummy page in TTM, use that one instead of allocating a
new one.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>