msm: mdss: support mdp idle power collapse only for cmd mode panels

With MDP idle power collapse feature enabled, MDSS GDSC gets turned off
whenever there are no updates sent to the panel which results in all
hardware registers getting reset. These hardware registers need to be
restored whenever on the next udpate. Currently, this restore
functionality is only supported for command mode panels. However it is
possible that there could be additional framebuffer devices that may be
active (such as a writeback device) in conjunction with the primary
panel. In such cases, if idle power collapse happens, then these control
paths may not get correctly restored leading to unexpected behaviors.
Fix this by preventing MDP idle power-collapse when such panels are
active.

Change-Id: I226e72086bb6232ae452a999d961c80f2841f833
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>