etnaviv: fix alpha blend with dither on older GPUs

etnaviv: fix alpha blend with dither on older GPUs

While setting up DITHER_MODE allows alpha blending to work properly
together with dithering on new GPUs (those with PE_DITHER_FIX), older
cores still change the render target. As dithering is optional and
implementation defined we can simply disable it on the affected GPUs,
when alpha blending is enabled to work around this bug.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13396>