mdss: Fix writeback

Fix retire fence threshold for writeback.
Notify when writeback buf is not available.

Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>
Signed-off-by: Naseer Ahmed <naseer@codeaurora.org>
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>
Signed-off-by: Iliyan Malchev <malchev@google.com>
msm: mdss: allow re-queue of writeback buffers

When a buffer has been properly processed, user space may choose to use
sync fence to acknowledge when buffer is complete. In such cases dequeue
is not not necessary, allow this use case by allowing client to re-queue
of writeback buffers as long as the buffer has been properly processed
already.

Change-Id: Ibac1eb9710477734e1ee2daf8136085d4439b0d4
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>