gbm/dri: Flush after unmap

gbm/dri: Flush after unmap

Drivers may queue dma operations on the context at unmap time so we need
to flush to make sure the data gets to the bo. Ideally the application
would take care of this, but since there appears to be no exported gbm
flush functionality we need to explicitly flush at unmap time.

This fixes a problem where kmscube on vmwgfx in rgba textured mode would
render using an uninitialized texture rather than the intended
rgba pattern.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>