freedreno: Make the slice pitch be bytes, not pixels.

Back in a2xx, HW pitches were in pixels, so storing that was reasonable.
Ever since then, the HW wants pitches in bytes, and we have only one
instance of using pitch in pixels in the code (a3xx sysmem path).

Flip things around so that only a2xx has to worry about the cpp for
looking at pitches.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4558>