nvk: Add a _pad field to nvk_cbuf
What does this MR do and why?
nvk: Add a _pad field to nvk_cbuf
We use nvk_cbuf as a key in a hash table in nvk_nir_lower_descriptors()
so we want to make sure it always gets fully initialized.
Fixes: f1c909edd5c9 ("nvk/nir: Add cbuf analysis to nvi_nir_lower_descriptors()")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/10956