drm/via: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of letting the code fall
through to the next case.

Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Sam Ravnborg <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/c2[e-mail]

drivers/gpu/drm/via/via_irq.c

···
 		irqwait->request.sequence +=
 			atomic_read(&cur_irq->irq_received);
 		irqwait->request.type &= ~_DRM_VBLANK_RELATIVE;
+		break;
 	case VIA_IRQ_ABSOLUTE:
 		break;
 	default: