drm/msm/dpu: remove unused variables new_cnt and old_cnt in dpu_encoder_phys_vid_vblank_irq()

We found two unused variables new_cnt and old_cnt when build kernel with
W=1.

So delete it.

Signed-off-by: Luo Jiaxing <luojiaxing@huawei.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>