drm/amdgpu: move keep stolen memory check into gmc core

Rather than leaving this as a gmc v9 specific hack.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>