Revert "draw: clear the draw buffers in draw"

Revert "draw: clear the draw buffers in draw"

This reverts commit 41966fdb3b71c0b70aeb095e0eb3c5626c144a3a.
While it's a lot cleaner it causes regressions because
the draw interface is always called from the draw functions
of the drivers (because the buffers need to be mapped) which
means that the stream output buffers endup being cleared on
every draw rather than on setting.

Signed-off-by: Zack Rusin <zackr@vmware.com>