mdss: display-port: fix Display-port disable sequence

Fix the OFF path for display-port driver when cable is
disconnected. Check for link clock status before accessing
any of the mainlink registers. Use common mutex for
DP_ON and DP_OFF sequence. Remove the resource vote
when PLL is diabled.

Change-Id: I9b81f79043b4ea7355b99ba9d8347d79bed10153
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>