msm: mdss: fix suspend coming to MDP before panel drivers

If suspend event is received by MDP before panels, MDP needs to ensure
other panels are off before turning MDP clocks off. Remove handlers from
framebuffer driver and handle all suspend logic within MDP driver to
handle this scenario.

Change-Id: Idcda2dd29b28a9993edca78b7e0778985e44b664
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>