drm/gma500: Add compat ioctl

Hook up drm_compat_ioctl to support 32-bit userspace on 64-bit kernels.
It turns out that N2600 and N2800 comes with 64-bit enabled. We
previously assumed there where no such systems out there.

Cc: stable@vger.kernel.org
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20161101144315.2955-1-patrik.r.jakobsson@gmail.com