display/exynos4210_fimd: Remove redundant statement in exynos4210_fimd_update()

Clang static code analyzer show warning:
hw/display/exynos4210_fimd.c:1313:17: warning: Value stored to 'is_dirty' is never read
                is_dirty = false;

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200302130715.29440-9-kuhn.chenqun@huawei.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

hw/display/exynos4210_fimd.c

@@ -1311,7 +1311,6 @@ static void exynos4210_fimd_update(void *opaque)
                 }
                 host_fb_addr += inc_size;
                 fb_line_addr += inc_size;
-                is_dirty = false;
             }
             g_free(snap);
             blend = true;