llvmpipe: eliminate last usage of step array in rast_tmp.h

For 16 and 64 pixel levels, calculate a mask which is linear in x and
y (ie not in the swizzle layout).

When iterating over full and partial masks, figure out position by
manipulating the bit number set in the mask, rather than relying on
postion arrays.

Similarly, calculate the lower-level c values from dcdx, dcdy and the
position rather than relying on the step array.

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