i965: Rename pixel_scoreboard_clear to last_render_target for clarity.

i965: Rename pixel_scoreboard_clear to last_render_target for clarity.

Finding this bit in the documentation proved challenging.  It wasn't in
the SEND instruction's message descriptor section, nor the data port
message descriptor section.  It turns out to be part of the Render
Target Write message's control bits, and in the documentation is named
"Last Render Target Select".

Shaders that use Multiple Render Targets should set this bit on the last
RT write, but not on any prior ones.

The GPU does update the Pixel Scoreboard appropriately, but doesn't
document this bit as directly causing a scoreboard clear.

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