mdss: dsi: Control MDSS bus clocks from within DSI driver

DSI driver needs to ensure that the MDSS AHB and AXI bus
clocks are enabled before accessing any registers or
sending DSI commands, to avoid potential unclocked register
accesses or DMA timeouts.

Change-Id: I69cf48d7f8228e8a15a6335912f712555cb641eb
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>