drm/amd/amdgpu: Return error if initiating read out of range on vram

BugLink: http://bugs.launchpad.net/bugs/1707233

commit 9156e723 upstream.

If you initiate a read that is out of the VRAM address space return
ENXIO instead of 0.

Reads that begin below that point will read upto the VRAM limit as
before.
Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

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

@@ -1126,6 +1126,9 @@ static ssize_t amdgpu_ttm_vram_read(struct file *f, char __user *buf,
 	if (size & 0x3 || *pos & 0x3)
 		return -EINVAL;
 
+	if (*pos >= adev->mc.mc_vram_size)
+		return -ENXIO;
+
 	while (size) {
 		unsigned long flags;
 		uint32_t value;