msm: mdss: fix fb device pm suspend and resume handlers

Under normal circumstances, the framebuffer device should have been
blanked or configured to a low power mode prior to the pm suspend
callback getting executed. In cases when this sequence is not followed,
blank the framebuffer as part of pm suspend handler to ensure that all
display resources are freed. Similarly, if the framebuffer was
explicitly blanked during suspend, then unblank it when resuming. This
also eliminates the need for the existing sysfs node to indicate
always-on display mode.

Change-Id: I1613f5eba420c9587af2ecb04bc5cb115b7a1095
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>