msm: mdss: fix NOC error in accessing accessing DMA status reg

In current implementation theres is a possibility that clk_off_work
will get scheduled twice after idle time out. When this happens
NOC error are observed in accessing DMA status register.
Fix issue by scheduling clk_off_work only once after each idle time out.

Change-Id: I4b6b2acb8f3260d281d5ca6ff9d54de7051e7b39
Signed-off-by: Abhijith Desai <desaia@codeaurora.org>