i965/vec4: Don't set header_present if texel offsets are all 0.

i965/vec4: Don't set header_present if texel offsets are all 0.

In theory, a shader might use textureOffset() but set all the texel
offsets to zero.  In that case, we don't actually need to set up the
message header - zero is the implicit default.

By moving the texture_offset setup before the header_present setup, we
can easily only set header_present when there are non-zero texel offset
values.

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