msm: mdss: refactor pipe retrieval and ref count logic

Allow only retrieval of pipes that are on particular framebuffer device
used list. This ensures that pipe is not modified from a different fb
device node and cause potential issues.

Also, modify the ref count logic to use kref and ensure pipe is freed
only while holding mutex in case there could be race condition that
pipe is allocated while being freed.

Change-Id: I626323b5df981a7ed1e03196126cd4376b5ba6a6
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>