iris: Mark cbuf0 as not needing uploading every single time

iris: Mark cbuf0 as not needing uploading every single time

I neglected to mark cbuf0_needs_upload = false after uploading it.
The obvious fix regressed user clip plane tests, because of a second
bug: we also forgot to mark that they may need re-uploading when
changing shader programs (which may have more or less system values).

Thanks to Timur Kristóf for catching the original issue.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>