freedreno/ir3: Merge the redundant immediate_idx/immediates_count fields

I got tripped up again with the index vs count vs size fields and I'd
rather we didn't store the redundant info.  Settle on immediates_count as
"how many dwords of immediates we have"

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5990>
7 files changed