panfrost: Promote blend shader outputs 8->16-bit

..on Bifrost and later, where the conversion hardware makes this reasonable.
This saves us from inserting a pile of conversions in the compiler to lower away
the 8-bit input/output. This also generates substantially better code.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
96 jobs for !17841 with panfrost/fp16-rendering-bifrost in 7 seconds (queued for 9 seconds)
merge request