drm/exynos/fbdev: don't set fix.smem/mmio_{start,len}

Kernel access to the eyxnos fbdev framebuffer is via its gem object's
kernel mapping (kvaddr, stored in info->screen_base).

User space access is provided by mmap(), read() and write() of /dev/fb/fb0.
These functions also only use screen_base/screen_size().

Therefore, it is not necessary to set fix->smem_{start,len} or
fix->mmio_{start,len} fields.

This avoids leaking kernel, physical and dma mapped addresses to user
space via the ioctls FBIOGET_VSCREENINFO and FBIOGET_FSCREENINFO.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>