main: Fix some "may be used uninitialized" warnings

/usr/include/glib-2.0/glib/gmacros.h: In function ‘get_drm_cards’:
/usr/include/glib-2.0/glib/gmacros.h:1212:17: warning: ‘parent’ may be used uninitialized [-Wmaybe-uninitialized]
 1212 |     { if (_ptr) (cleanup) ((ParentName *) _ptr); }                                                              \
      |                 ^
../src/switcheroo-control.c:367:32: note: ‘parent’ was declared here
  367 |         g_autoptr(GUdevDevice) parent;
      |                                ^~~~~~
/usr/include/glib-2.0/glib/gmacros.h:1212:17: warning: ‘parent’ may be used uninitialized [-Wmaybe-uninitialized]
 1212 |     { if (_ptr) (cleanup) ((ParentName *) _ptr); }                                                              \
      |                 ^
../src/switcheroo-control.c:339:32: note: ‘parent’ was declared here
  339 |         g_autoptr(GUdevDevice) parent;
      |                                ^~~~~~
/usr/include/glib-2.0/glib/gmacros.h:1212:17: warning: ‘parent’ may be used uninitialized [-Wmaybe-uninitialized]
 1212 |     { if (_ptr) (cleanup) ((ParentName *) _ptr); }                                                              \
      |                 ^
../src/switcheroo-control.c:289:32: note: ‘parent’ was declared here
  289 |         g_autoptr(GUdevDevice) parent;
      |                                ^~~~~~
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