radv: correctly skip vertex loads with packed formats

radv: correctly skip vertex loads with packed formats

This is supposed to check whether any channels of the element is used, but
skipped_start is always 0 for packed formats.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Fixes: a46acdbc3ff ("radv: Lower non-dynamic VS inputs in NIR.")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/9062
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23370>