drm/amdgpu: Free VGA stolen memory as soon as possible.

Reserved VRAM is used to avoid overriding pre OS FB.
Once our display stack takes over we don't need the reserved
VRAM anymore.

v2:
Remove comment, we know actually why we need to reserve the stolen VRAM.
Fix return type for amdgpu_ttm_late_init.
v3:
Return 0 in amdgpu_bo_late_init, rebase on changes to previous patch
v4: rebase
v5:
For GMC9 reserve always just 9M and keep the stolem memory around
until GART table curruption on S3 resume is resolved.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>