drm/amdgpu/pp/smu7: cache smu firmware toc

Rather than calculating it everytime we rebuild the toc
buffer, calculate it once initially and then just copy
the cached results to the vram buffer.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>