Use materialize SRF mode in brin_page_items

Use materialize SRF mode in brin_page_items

This function was using the single-value-per-call mechanism, but the
code relied on a relcache entry that wasn't kept open across calls.
This manifested as weird errors in buildfarm during the short time that
the "brin-1" isolation test lived.

Backpatch to 9.5, where it was introduced.