msm: mdss: fix extra refcount in the mdp clock during clock gate

There is a race condition between kickoff and early
wakeup events; add mutex to protect the state of the
resources and prevent race condition.

Change-Id: Ibf3284f1d1fce26b88ea0a1f56f42a4682b16c77
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>