drm: Convert cmpxchg(bool) back to a two step operation

ARM v6 (at least) only allows cmpxchg on 32bit variables which doesn't
always include the bool type.

drivers/built-in.o: In function `vblank_disable_and_save':
imx-ocotp.c:(.text+0xb45e8): undefined reference to `__bad_cmpxchg'
Makefile:986: recipe for target 'vmlinux' failed

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reported-by: Leonard Crestez <leonard.crestez@nxp.com>
Fixes: 43dc7fe2b211 ("drm: Mark up accesses of vblank->enabled outside of its spinlock")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170330140832.32377-1-chris@chris-wilson.co.uk

drivers/gpu/drm/drm_irq.c

@@ -338,8 +338,10 @@ static void vblank_disable_and_save(struct drm_device *dev, unsigned int pipe)
 	 * calling the ->disable_vblank() operation in atomic context with the
 	 * hardware potentially runtime suspended.
 	 */
-	if (cmpxchg_relaxed(&vblank->enabled, true, false))
+	if (vblank->enabled) {
 		__disable_vblank(dev, pipe);
+		vblank->enabled = false;
+	}
 
 	/*
 	 * Always update the count and timestamp to maintain the