Commit [0] stopped setting fix.smem_start and fix.smem_len when creating
the fbdev.

[0] 2f1eab8d
  drm/exynos/fbdev: don't set fix.smem/mmio_{start,len}

However, smem_len is used by some userland applications to calculate the
size for mmap.  In particular, it is used by xf86-video-fbdev:

http://cgit.freedesktop.org/xorg/xserver/tree/hw/xfree86/fbdevhw/fbdevhw.c?id=xorg-server-1.15.99.903#n571

So, let's restore setting the smem_len to unbreak things for these users.

Note: we are still leaving smem_start set to 0.
Reported-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Reported-by: Gunther Noack <me@guenthernoack.de>
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>