ARM: dts: msm: Enable dynamic clock gating for MDSS AHB clock

On APQ8084, currently AHB clock is always on from device tree settings.
With this change, it will be enabled if BWC decode is enabled and source
dimensions are changed. Since the source dimensions are double buffered,
the MDP AHB clock will have to be forced on before writing to the source
dimension registers and then the force on turned off after the changes
have been applied (ie next vsync).

Change-Id: Iaf4cae47512c316d647dafb15931a2e8b752315e
Signed-off-by: Shalabh Jain <shalabhj@codeaurora.org>