freedreno/decode: try harder to not crash in disasm

Move the handling for catching asserts when we start decoding garbage
into disasm-a3xx.  This way it can also cover other cases where cffdec
tries to disassemble memory, such as SP_xS_OBJ_START.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6242>