drm/amd/amdgpu: fix offset into page with amdgpu_iomem debugfs file

The offset inside the page wasn't included in the copy call meaning
the start of the page was being read/written instead.
Reported-by: NJay Cornwall <Jay.Cornwall@amd.com>
Signed-off-by: NTom St Denis <tom.stdenis@amd.com>
Reviewed-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c

@@ -2021,7 +2021,7 @@ static ssize_t amdgpu_iomem_read(struct file *f, char __user *buf,
 			return -EPERM;
 
 		ptr = kmap(p);
-		r = copy_to_user(buf, ptr, bytes);
+		r = copy_to_user(buf, ptr + off, bytes);
 		kunmap(p);
 		if (r)
 			return -EFAULT;
@@ -2065,7 +2065,7 @@ static ssize_t amdgpu_iomem_write(struct file *f, const char __user *buf,
 			return -EPERM;
 
 		ptr = kmap(p);
-		r = copy_from_user(ptr, buf, bytes);
+		r = copy_from_user(ptr + off, buf, bytes);
 		kunmap(p);
 		if (r)
 			return -EFAULT;