panfrost: Allow pixels using discard to be killed

info.fs.sidefx considers discard() to be a side effect. That definition is...
dubious at best. It certainly isn't the definition needed for forward pixel
kill. The only reason pixels couldn't be killed by FPK is if the shader has side
effects in the sense of writing to memory. Use that more precise condition so
FPK works more often.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Closes: #5607
Part-of: <!16984>
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