drm/msm/dpu: Add mutex lock in control vblank irq

In the Linux kernel, the following vulnerability has been resolved: drm/msm/dpu: Add mutex lock in control vblank irq Add a mutex lock to control vblank irq to synchronize vblank enable/disable operations happening from different threads to prevent race conditions while registering/unregistering the vblank irq callback. v4: -Removed vblank_ctl_lock from dpu_encoder_virt, so it is only a parameter of dpu_encoder_phys. -Switch from atomic refcnt to a simple int counter as mutex has now been added v3: Mistakenly did not change wording in last version. It is done now. v2: Slightly changed wording of commit message Patchwork: https://patchwork.freedesktop.org/patch/571854/