lib/gem_engine_topology: Fix broken compare of device links

When looking for a primary counterpart of a render device, we compare
"device" links of both nodes.  If those links point to the same device
then we know we've found the correct primary node.

However, readlinkat() function we use doesn't explicitly terminate read in
strings with null characters, and then the comparison occasionally fails.

Process the second (potential primary counterpart) node only if its
"device" link is of the same length as that of the render node, and limit
the number of compared characters to that length.

Closes: drm/i915/kernel#6268


Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>