drm/tilcdc: ensure nonatomic iowrite64 is not used

Add a check to ensure iowrite64 is only used if it is atomic.

It was decided in [1] that the tilcdc driver should not be using an
atomic operation (so it was left out of this patchset). However, it turns
out that through the drm code, a nonatomic header is actually included:

include/linux/io-64-nonatomic-lo-hi.h
is included from include/drm/drm_os_linux.h:9:0,
from include/drm/drmP.h:74,
from include/drm/drm_modeset_helper.h:26,
from include/drm/drm_atomic_helper.h:33,
from drivers/gpu/drm/tilcdc/tilcdc_crtc.c:19:

And thus, without this change, this patchset would inadvertantly
change the behaviour of the tilcdc driver.

[1] lkml.kernel.org/r/[e-mail]

Signed-off-by: Logan Gunthorpe <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Cc: Jyri Sarha <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Tomi Valkeinen <[e-mail]>
Cc: David Airlie <[e-mail]>
Signed-off-by: Jyri Sarha <[e-mail]>

drivers/gpu/drm/tilcdc/tilcdc_regs.h

···
 	struct tilcdc_drm_private *priv = dev->dev_private;
 	volatile void __iomem *addr = priv->mmio + reg;
 
-#ifdef iowrite64
+#if defined(iowrite64) && !defined(iowrite64_is_nonatomic)
 	iowrite64(data, addr);
 #else
 	__iowmb();