drm/msm/sde: fix race condition in vblank control interrupts

When register/deregister of interrupts happen across multiple threads,
there can be a race condition where irq idx can be in invalid state.
Fixed it with adding mutex lock to avoid the race condition.

Change-Id: Ic95d0fc4c4890cf6d98aa677aa65aca1adc93d54
Signed-off-by: Raviteja Tamatam <travitej@codeaurora.org>