radv: improve gathering of load_push_constants with dynamic bindings

radv: improve gathering of load_push_constants with dynamic bindings

For example, if a pipeline has two stages VS and FS. And if only
the fragment stage needs dynamic bindings, we shouldn't allocate
an extra user SGPR for the vertex stage. Of course, if the vertex
stage loads constants, it needs an user SGPR.

This should reduce the number of SET_SH_REG packets that are emitted.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>