i965: Restore a lost comment about TXF offset bugs.

Eric's original code to work around TXF offset bugs contained a comment
explaining the problem, which was lost when Chris generalized it to an
IR transformation (in commit 598ca510b8a118c3c7e18b5d031a2b116120e0a6).

This commit adds the original comment to the newer code.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
1 file changed