mesa: Also set the remaining draw buffers to GL_NONE when updating just the first...

mesa: Also set the remaining draw buffers to GL_NONE when updating just the first buffer in _mesa_drawbuffers().

Without this we'd miss the last update in a sequence like {COLOR0, COLOR1},
{COLOR0}, {COLOR0, COLOR1}. I originally had a patch for this that called
updated_drawbuffers() when the buffer count changed, but later realized that
was wrong. The ARB_draw_buffers spec explicitly says "The draw buffer for
output colors beyond <n> is set to NONE.", and this is queryable state.
This fixes piglit arb_draw_buffers-state_change.

NOTE: This is a candidate for the 7.11 branch.

Signed-off-by: Henri Verbeet <hverbeet@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>