ARM: dts: msm: add mdss gdsc supply to dsi controller device

It is possible for the DSI controller to be active when MDP is
power collapsed. DSI controller needs to have it's own vote for
mdss gdsc to ensure that gdsc remains on in such cases.

Change-Id: I37f98c6e4f6d30908373b812fc50e29ba001b752
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>