drm/amdgpu/pp/smu7: use a local variable for toc indexing

Rather than using the index variable stored in vram.  If
the device fails to come back online after a resume cycle,
reads from vram will return all 1s which will cause a
segfault. Based on a patch from Thomas Martitz <kugel@rockbox.org>.
This avoids the segfault, but we still need to sort out
why the GPU does not come back online after a resume.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=105760
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org