i965: Share the draw x/y offset masking code between main/blorp and all gens.

i965: Share the draw x/y offset masking code between main/blorp and all gens.

This code is twisty, and the comment before most of the blocks was actually
giving me the opposite impression from its intention: We want to apply as much
of our offset as possible through coarse tile-aligned adjustment, since we can
do so independently per buffer, and apply the minimum we can through
fine-grained drawing offset x/y, since it has to agree between all buffers.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>