i965/fs: Move message header and texture offset setup to generate_tex().

i965/fs: Move message header and texture offset setup to generate_tex().

Setting the texture offset bits in the message header involves very
specific hardware register descriptions.  As such, I feel it's better
suited for the lower level "generate" layer that has direct access to
the weird register layouts, rather than at the fs_inst abstraction layer.

This also parallels the approach I took in the VS backend.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>