radeon: Fix flushing before writing a teximage's BO when !t->bo.

radeon: Fix flushing before writing a teximage's BO when !t->bo.

Before, if we ended up here without a BO for our image, but did choose
a miptree that had active rendering in the command buffer, our
teximage data would jump ahead of the rendering using the old texture
contents.

This showed up as breakage in gen-teximage and friends in the
following commit.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>