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
Signed-off-by: Paloma Arellano <quic_parellan@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/571854/
Link: https://lore.kernel.org/r/20231212231101.9240-2-quic_parellan@quicinc.comSigned-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>