drm: exynos: mixer: fix using usleep() in atomic context

This patch fixes calling usleep_range() after taking reg_slock
using spin_lock_irqsave(). The mdelay() is used instead.
Waiting in atomic context is not the best idea in general.
Hopefully, waiting occurs only when Video Processor fails
to reset correctly.
Signed-off-by: NTomasz Stanislawski <t.stanislaws@samsung.com>
Signed-off-by: NInki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/exynos_mixer.c

@@ -652,7 +652,7 @@ static void vp_win_reset(struct mixer_context *ctx)
 		/* waiting until VP_SRESET_PROCESSING is 0 */
 		if (~vp_reg_read(res, VP_SRESET) & VP_SRESET_PROCESSING)
 			break;
-		usleep_range(10000, 12000);
+		mdelay(10);
 	}
 	WARN(tries == 0, "failed to reset Video Processor\n");
 }