msm: mdss: Add support to selectively control bus and link clocks

DSI driver needs two different types of clocks for normal
operation - bus clocks and link clocks. The clock control function
enables/disables both these set of clocks together. However,
under certain circumstances, it is required to control just the
link clocks or the bus clocks. To support this, add separate
ref counts for link and bus clocks and modify the clock control
function to selectively control these clocks.

Change-Id: I25312131fc737b56dc15f4edf5e55a5824dad855
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>