frontends/va: Only increment slice offset after first slice parameters
What does this MR do and why?
frontends/va: Only increment slice offset after first slice parameters
Fixes slice offset if app submits exactly one data buffer followed by
parameter buffers.
Fixes: 6746d4df6ea ("frontends/va: Fix AV1 slice_data_offset with multiple slice data buffers")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/11133