i915: Refine the texture indirect lookup accounting.

i915: Refine the texture indirect lookup accounting.

Without this, we would reject programs which sampled multiple times from
registers defined in the same phase (block of instructions with the same
texture indirection count), as each sample would count as a new phase
beginning.  Instead, keep track of which phases registers were written in,
and only bump phase when we're reading from one generated in this phase.

On the other hand, we failed to count oC or oD texture samples as being new
phases.

Bug #17865.