disp: msm: sde: disable idle notification

This change disables sending idle notification if system
cache is already enabled. If system cache is enabled it
establishes that driver has already send prior
notification and no configuration change was requested.

Change-Id: I1aee002ab3c8c3d4193a8e7a4890d8e4f24da804
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>