nir: xfb_buffer_info::stride is in bytes

This fixes regressions found by @pepp in !16750 (merged).

For the NIR XFB gathering as well as all the Vulkan drivers, buffer strides in nir_xfb_info are in bytes. When Marek started using nir_xfb_info for GLSL on radeonsi, he copied directly from the GLSL struct which has strides in dwords. This inconsistency didn't show up until I went through and started us using the NIR passes for GL drivers directly without going through the GLSL structs. We could change the nir_xfb_buffer_info field to be in dwords to be consistent with shader_info but that would mean changing all the Vulkan drivers but, for now, it's easier to always use bytes in nir_xfb_info.