x11-wm/cage: fix DRM session after r521656

After VT switch nothing was rendered. From debug log before/after:

[backend/drm/drm.c:1485] Requesting modeset for 'DP-1'
+[backend/drm/drm.c:697] Modesetting 'DP-1' with '3840x2160@60000 mHz'
+[backend/drm/drm.c:592] Initializing renderer on connector 'DP-1'
+[GLES2] FS SIMD8 shader: 5 inst, 0 loops, 24 cycles, 0:0 spills:fills, Promoted 0 constants, compacted 80 to 48 bytes.
+[GLES2] FS SIMD16 shader: 5 inst, 0 loops, 34 cycles, 0:0 spills:fills, Promoted 0 constants, compacted 80 to 48 bytes.
+[GLES2] VS SIMD8 shader: 28 inst, 0 loops, 116 cycles, 0:0 spills:fills, Promoted 0 constants, compacted 448 to 336 bytes.
+[GLES2] FS SIMD16 shader: 2 inst, 0 loops, 0 cycles, 0:0 spills:fills, Promoted 0 constants, compacted 32 to 32 bytes.