pvr: remove pvr_pbe_get_src_pos()

This should have been removed as part of f59c6eef168 ("pvr: Move PBE START_POS
into csb enum helpers header").

Signed-off-by: Frank Binns <frank.binns@imgtec.com>
Reviewed-by: Karmjit Mahil <Karmjit.Mahil@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25328>