i965: Use sendc for all render target writes on Gen6+.

The sendc instruction causes the fragment shader thread to wait for
any dependent threads (i.e. threads rendering to overlapping pixels)
to complete before sending the message.  We need to use sendc on the
first render target write in order to guarantee that fragment shader
outputs are written to the render target in the correct order.

Previously, we only used the "sendc" instruction when writing to
binding table index 0.  This did the right thing for fragment shaders,
because our fragment shader back-ends always issue their first render
target write to binding table index 0.  However, it did the wrong
thing for blorp, which performs its render target writes to binding
table index 1.

A more robust solution is to use sendc for all render target writes.
This should not produce any performance penalty, since after the first
sendc, all of the dependent threads will have completed.

For more information about sendc, see the Ivy Bridge PRM, Vol4 Part3
p218 (sendc - Conditional Send Message), and p54 (TDR Registers).

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