i965/fs: Enable rounding mode on f2f16 ops

i965/fs: Enable rounding mode on f2f16 ops

By default we don't set the rounding mode. We only set
round-to-near-even or round-to-zero mode if explicitly set from nir.

v2: Use a single SHADER_OPCODE_RND_MODE opcode taking an immediate
    with the rounding mode (Curro)

v3: Use new helper brw_rnd_mode_from_nir_op  (Jason Ekstrand)

Signed-off-by: Jose Maria Casanova Crespo <jmcasanova@igalia.com>
Signed-off-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>