pvr: VDMCTRL_INDEX_LIST[5-9] are not needed

An investigation turned up no need for these values on the currently
supported hardware configurations, so remove this comment for now.

Signed-off-by: Matt Coster <matt.coster@imgtec.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19673>