msm: mdss: improve mdp buffers management

Currently buffers are unmapped as soon as it's queued to MDP, however
the buffer still belongs to MDP until a new buffer is pushed.
Change current buffer managment to track of buffers currently being
displayed and properly unmap buffers after a new buffer is received.

Change-Id: I6932191fc787f6f5b673edfb690ce1317e283587
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>