msm: mdss: hold ref count on pm runtime during clock control

In order to ensure that MDSS gdsc is enabled while MDP clocks are
running keep a ref count on PM runtime while clocks are enabled.
PM runtime logic will take care of disabling MDSS gdsc once all
references are released.

Change-Id: Ieabc100609ef4a12d50d69137def4571a53d4f30
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>