msm: mdss: unmap user writeback buffers after session is terminated

Currently if buffers are provided to writeback driver through user space
ioctl, these are never unmapped and may cause leaks in memory. Also, we
can reuse the nodes which are already mapped until session is
terminated.

Change-Id: Ic786dfa735af529998bfc9467ac872b2badc0e31
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>