freedreno/cffdec: When .mergedregs is set, don't count half regs.

This matches what ir3.c does in the mergedregs case: just count max full
reg used.  This flag is unset so far, but will be soon and keeps our
output comparable between blob and freedreno.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6323>
1 file changed