pan/midgard: Fix recursive csel scheduling

pan/midgard: Fix recursive csel scheduling

Corner case causing invalid scheduling on shaders with nested csels,
i.e. GLSL code resembling:

   (foo ? bool1 : bool2) ? x : y

By explicitly disallowing csels this is fixed.

Fixes INSTR_INVALID_ENC on a glamor shader (noticeable with slowdown and
visual corruption when scrolling "too far" on GTK apps).

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3463>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3463>