v3d/compiler: add v3dv_prog_data_size helper

Main use case is to help to implement Vulkan PipelineCache, as we are
serializing/deserializing the prog_data too.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6078>
2 files changed