msm: mdss: fix handling of runtime pm ref counts for fb device

In the current implementation, pm runtime status for the framebuffer
device changes to active when the overlay session is started. With idle
power collapse feature enabled, it is possible that the MDSS core could
be turned off prior to starting the overlay session depending on the
duration of the idle timeout. This may result in an invalid hardware
configuration leading to various target instabilities. Fix this by
holding a reference to the pm runtime when mdp overlay is turned on.
Similarly, when the userspace process is killed, wake up MDSS from idle
power collapse before shutting down the fb device.

Change-Id: Ie7748732fe76f9b2766da32b972e1e9fe7ff8751
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>