drm/msm: remove variable set but not used

drm/msm: remove variable set but not used

The code that uses variable mdss has been removed, So the declaration
and assignment of the variable can be removed.

Eliminate the following clang warning:
drivers/gpu/drm/msm/msm_drv.c:513:19: warning: variable 'mdss' set but
not used [-Wunused-but-set-variable]

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Fixes: dd7335016cd7 ("drm/msm: Initialize MDSS irq domain at probe time")
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20211216031103.34146-1-yang.lee@linux.alibaba.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>