msm: mdss: Avoid mutex lockup in dynamic switching

Calling blank/unblank after locking the backlight mutex will result
in mutex lockup as blank/unblank would again try to aquire the same
lock. Avoid this situation in Dynamic Switching.

Change-Id: I7ddb2afec605f236071976420e0d6bd0c707e4d0
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>