drm/vc4: Match drm_dev_enter and exit calls in vc4_hvs_atomic_flush

Commit 92c17d1 ("drm/vc4: hvs: Ignore atomic_flush if we're disabled")
added a path which returned early without having called drm_dev_exit.

Ensure all paths call drm_dev_exit.

Fixes: 92c17d1 ("drm/vc4: hvs: Ignore atomic_flush if we're disabled")
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241008-drm-vc4-fixes-v1-2-9d0396ca9f42@raspberrypi.com
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>

drivers/gpu/drm/vc4/vc4_hvs.c

@@ -603,7 +603,7 @@ void vc4_hvs_atomic_flush(struct drm_crtc *crtc,
 	}
 
 	if (vc4_state->assigned_channel == VC4_HVS_CHANNEL_DISABLED)
-		return;
+		goto exit;
 
 	if (debug_dump_regs) {
 		DRM_INFO("CRTC %d HVS before:\n", drm_crtc_index(crtc));
@@ -686,6 +686,7 @@ void vc4_hvs_atomic_flush(struct drm_crtc *crtc,
 		vc4_hvs_dump_state(hvs);
 	}
 
+exit:
 	drm_dev_exit(idx);
 }