etnaviv: switch to S_FIXED(..) macro

etnaviv: switch to S_FIXED(..) macro

In different traces for different GPU models I see the same pattern:
    0x00c80000, /*   [00A00] PA.VIEWPORT_SCALE_X = 200.000000 */
    0xff880000, /*   [00A04] PA.VIEWPORT_SCALE_Y = -120.000000 */

etna_f32_to_fixp16(..) handles the negative case differently then blob. In the
concrete example -120 gets converted to 0xff880001. Switch to S_FIXED(..) to
simplify our code and to get the same results as blob.

Signed-off-by: Christian Gmeiner <cgmeiner@igalia.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24364>