drm/amdgpu: used cached pcie gen info for SI (v2)

Rather than querying it every time we need it.
Also fixes a crash in VM pass through if there is no
root bridge because the cached value fetch already checks
this properly.

v2: fix includes

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=105244


Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Rex <Zhu&lt;rezhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org