xf86drm: Fix error handling for drmGetDevices()

If the opendir() call in drmGetDevices() returns failure, we jump to an
error label that calls closedir() and then returns.  However this means
that we're calling closedir(NULL) which may not be safe on all
implementations.  We are also leaking the local_devices array that was
allocated before the opendir() call.

Fix both of these issues by jumping to an earlier error label (to free
local_devices) and guarding the closedir() call with a NULL test.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
[Emil Velikov: make the teardown symmetrical, remove the NULL check]
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>