freedreno/ir3: Don't force RTNE if rounding mode is undefined

Forcing round-to-nearest-even results in loss of opportunities for conversion folding, causing a regression in gfxbench gl_alu2.

Fixes: de195671 ("ir3: nir_op_f2f16 should round to even") Signed-off-by: Rob Clark robdclark@chromium.org