drm/amdgpu: move stolen vga bo from amdgpu to amdgpu.gmc

Since that is where we store the other data related to
the stolen vga memory.

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>