freedreno/rnn: Support stripes in rnndec_decodereg

freedreno/rnn: Support stripes in rnndec_decodereg

We'll need this for finding where INDIRECT/STRIDE are in
CP_DRAW_INDIRECT_MULTI, since they are in different locations in each
variant.

This is tricky because we need to bubble up success/failure to the upper
levels, and 0 could be a valid offset if the stripe is inside an array
or in a packet. Hence we refactor tryreg to return success/failure
separately, although I stopped short of modifying rnndec_decodereg
itself.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6104>