drm/msm/mdp5: Remove set but not used variable 'hw_cfg' in modeset_init

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

drivers/gpu/drm/msm/disp/mdp5/mdp5_kms.c: In function modeset_init:
drivers/gpu/drm/msm/disp/mdp5/mdp5_kms.c:458:28: warning: variable hw_cfg set but not used [-Wunused-but-set-variable]

It is not used since commit 36d1364a ("drm/msm/mdp5:
Clean up interface assignment")

Reported-by: Hulk Robot <hulkci@huawei.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1570630403-92371-1-git-send-email-zhengbin13@huawei.com