v3d: acquire scoreboard lock before first tlb read

Until now we have always been emitting our scoreboard locks on the last thread
switch to improve parallelism. We did this by emitting our last thread switch
right before our tlb writes at the very end of the program, where we know that
we are outside control flow.

Unfortunately, this strategy is not valid when we have tlb color reads too, as
these will happen before this point in the program and can happen inside
control flow.

To fix this we always emit a thread switch before the first tlb load and if we
see additional thread switches after that point, we change the strategy to lock
on the first thread switch.

v2: change the solution so it is expected to work in more scenarios (Eric).

Reviewed-by: Eric Anholt <[email protected]>

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