drm/msm/dpu: Remove some set but not used variables

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

drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c: In function _dpu_debugfs_show_regset32:
drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c:142:26: warning: variable priv set but not used [-Wunused-but-set-variable]
drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c: In function dpu_kms_prepare_commit:
drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c:271:21: warning: variable dev set but not used [-Wunused-but-set-variable]
drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c: In function _dpu_kms_hw_destroy:
drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c:555:21: warning: variable dev set but not used [-Wunused-but-set-variable]
drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c: In function dpu_kms_hw_init:
drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c:763:26: warning: variable priv set but not used [-Wunused-but-set-variable]
drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c: In function dpu_runtime_suspend:
drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c:1021:21: warning: variable ddev set but not used [-Wunused-but-set-variable]

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