drm/via: Fix vblank IRQ on VIA hardware.

via_enable_vblank wasn't setting the VBlank enable bit - instead, it
was masking out the rest of the register.

At the same time, fix via_disable_vblank to clear the VBlank enable
bit.
Signed-off-by: Dave Airlie <airlied@linux.ie>

drivers/gpu/drm/via/via_irq.c

@@ -183,7 +183,7 @@ int via_enable_vblank(struct drm_device *dev, int crtc)
 	}
 
 	status = VIA_READ(VIA_REG_INTERRUPT);
-	VIA_WRITE(VIA_REG_INTERRUPT, status & VIA_IRQ_VBLANK_ENABLE);
+	VIA_WRITE(VIA_REG_INTERRUPT, status | VIA_IRQ_VBLANK_ENABLE);
 
 	VIA_WRITE8(0x83d4, 0x11);
 	VIA_WRITE8(0x83d5, VIA_READ8(0x83d5) | 0x30);
@@ -194,6 +194,10 @@ int via_enable_vblank(struct drm_device *dev, int crtc)
 void via_disable_vblank(struct drm_device *dev, int crtc)
 {
 	drm_via_private_t *dev_priv = dev->dev_private;
+	u32 status;
+
+	status = VIA_READ(VIA_REG_INTERRUPT);
+	VIA_WRITE(VIA_REG_INTERRUPT, status & ~VIA_IRQ_VBLANK_ENABLE);
 
 	VIA_WRITE8(0x83d4, 0x11);
 	VIA_WRITE8(0x83d5, VIA_READ8(0x83d5) & ~0x30);