drm/radeon: protect concurrent smc register access with a spinlock

smc registers are access indirectly via the main mmio aperture, so
there may be problems with concurrent access.  This adds a spinlock
to protect access to this register space.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_device.c

@@ -1249,6 +1249,7 @@ int radeon_device_init(struct radeon_device *rdev,
 	/* Registers mapping */
 	/* TODO: block userspace mapping of io register */
 	spin_lock_init(&rdev->mmio_idx_lock);
+	spin_lock_init(&rdev->smc_idx_lock);
 	if (rdev->family >= CHIP_BONAIRE) {
 		rdev->rmmio_base = pci_resource_start(rdev->pdev, 5);
 		rdev->rmmio_size = pci_resource_len(rdev->pdev, 5);