drm/radeon: keep __user during cast

Silence static checker warning.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_ttm.c

@@ -923,7 +923,7 @@ static ssize_t radeon_ttm_vram_read(struct file *f, char __user *buf,
 		value = RREG32(RADEON_MM_DATA);
 		spin_unlock_irqrestore(&rdev->mmio_idx_lock, flags);
 
-		r = put_user(value, (uint32_t *)buf);
+		r = put_user(value, (uint32_t __user *)buf);
 		if (r)
 			return r;