draw: clear the draw buffers in draw

draw: clear the draw buffers in draw

Moves clearing of the draw so target buffers to the draw
module. They had to be cleared in the drivers before
which was quite messy.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>