msm: mdss: enable ref count based PM_QOS request for DSI

DSI controller requests for PM_QOS while sending DCS commands, so
that CPU does not go to idle state until DCS command transfer is
finished. But this request can come independently from multiple
clients such as different DSI controllers. So this change adds
ref count based PM_QOS request to avoid warning for multiple QOS
add request and unwanted results, if one client removes the request
while the other clients still need it.

Change-Id: If3ca0161923fcd73fdde77984aa5d80bfaec79a1
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>