v3d: Fix fbfetch with discards.

We can't do earlyz_with_discard when using fbfetch as we can have TLB reads
that occur before the discard. This can result in implicit Z writes which make
the setmsf instruction emitted as a result of the discard invalid.

Fixes: 332b3135 ("v3d: enable framebuffer fetch")

Reviewed-by: Jose Maria Casanova Crespo <jmcasanova@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <!34151>