drm/amdgpu/gmc9: don't keep stolen memory on Raven

drm/amdgpu/gmc9: don't keep stolen memory on Raven

Raven does not appear to be affected by the same issue
as vega10.  Enable the full stolen memory handling on
Raven.  Reserve the appropriate size at init time to avoid
display artifacts and then free it at the end of init once
the new FB is up and running.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=106639
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>