drm/amdgpu: add support for extended stolen vga memory

This will allow us to split the allocation for systems
where we have to keep the stolen memory around to avoid
S3 issues.  This way we don't waste as much memory and
still avoid any screen artifacts during the bios to
driver transition.

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>