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> Part-of: <mesa/mesa!10773>