panfrost: Implement sRGB blend shaders

panfrost: Implement sRGB blend shaders

We use the lowering in nir_format_convert. There are native ops for this
so this is far from optimal and not remotely efficient but as with most
blend shader things right now, it's hard enough to get it working, so
let's focus on that for now. We'll make it fast later (once we have
GLES3 stable, we can start optimizing these things).

Fixes dEQP-GLES3.functional.fragment_ops.blend.fbo_srgb.*

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>