drm/amdgpu: rename vram_scratch into mem_scratch

Rename vram_scratch into mem_scratch and allow allocating it into GTT as
well.

The only problem with that is that we won't have a default page for the
system aperture any more.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>