hexagon: dma optimizations (mostly fixing regressions) (llama/21137)

hexagon: dma optimizations (mostly fixing regressions) (llama/21137)

* hex-fa: add simple dma cache for Mask

I noticed that we were refetch the mask rows over and over.
This simple cache avoids that.

* hex-dma: unset in-order desc bit which caused signficant perf regression

We don't rely on true in order processing of the DMA descriptors anywhere.
Turns out this mode caused significant regression of around 3-4 TPS during token gen.

* hex-rope: update comment to clarify that we don't need in-order DMA completions