drm/msm/dpu: Remove set but not used variables 'mode', 'dpu_kms', 'priv'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c: In function dpu_encoder_virt_disable:
drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c:1199:27: warning: variable mode set but not used [-Wunused-but-set-variable]
drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c: In function _dpu_encoder_init_debugfs:
drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c:1963:18: warning: variable dpu_kms set but not used [-Wunused-but-set-variable]
drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c: In function dpu_encoder_frame_done_timeout:
drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c:2183:26: warning: variable priv set but not used [-Wunused-but-set-variable]

They are not used since commit 25fdd593 ("drm/msm:
Add SDM845 DPU support")

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>