drm/msm/dpu: Add callback function pointer check before its call

In dpu_core_irq_callback_handler() callback function pointer is compared to NULL,
but then callback function is unconditionally called by this pointer.
Fix this bug by adding conditional return.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: c929ac60 ("drm/msm/dpu: allow just single IRQ callback")
Signed-off-by: Aleksandr Mishin <amishin@t-argos.ru>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/588237/
Link: https://lore.kernel.org/r/20240408085523.12231-1-amishin@t-argos.ru


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>