i965/fs: Refactor sampler message header to duplicate less code.

i965/fs: Refactor sampler message header to duplicate less code.

Previously, the code to copy g0 to the message header existed in two
places - one for the texture offset case, and one for any other case.

By treating texture_offset as a special case of header_present, we can
remove this duplication and shorten the code.  Future patches which add
new header fields also won't have to add additional duplication.

This also clarifies a confusing construct.  The old code contained:

   } else if (inst->header_present) {
      if (brw->gen >= 7) {
         ...explicit copy from g0 to the message header...
      } else {
         /* Set up an implied move from g0 to the MRF. */
      }
   }

This looks like it might set up an implied move on Sandybridge, which
doesn't support those.  However, Sandybridge only uses a message header
for texture offsets, so it would never hit this code path.  The new code
avoids this implicit knowledge by only setting up an implied move on
Gen4-5.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>