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

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

drivers/gpu/drm/msm/disp/mdp5/mdp5_crtc.c: In function blend_setup:
drivers/gpu/drm/msm/disp/mdp5/mdp5_crtc.c:225:28: warning: variable hw_cfg set but not used [-Wunused-but-set-variable]

It is not used since commit 14be3200 ("drm/msm: rename mdp->disp")

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1570690506-83287-3-git-send-email-zhengbin13@huawei.com