v3dv/pipeline: use new helper vk_shader_module_to_nir

v3dv/pipeline: use new helper vk_shader_module_to_nir

In addition to use the helper, we also remove some of the lowering we
had at preprocess_nir, as they are called now by the helper.

As we are here we also move the call to nir_lower_sysvals_to_varyings,
that for some reason we were calling it before preprocess_nir.

It is worth to note that with this change we lose the ability to debug
the NIR just after spirv_to_nir using V3D_DEBUG, as now this is done
on vk_spirv_to_nir, and as mentioned that includes several lowerings
now. The workaround to that is to use NIR_DEBUG.

We also needed to change how to check the entrypoint on the broadcom
compiler, checking just if it is an entrypoint, instead of assuming
that the name will be "main".

v2: tweak comment, squash v3dv and compiler change (Iago)

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15449>