drm: avoid exposing kernel stack in compat_drm_getstats

The C standard does not specify the size of the integer used
to store an enum. Hence in structure drm_stats32_t alignment
bytes may exist.

To avoid exposing bytes from the kernel stack it is
necessary to initialize variable s32 completely.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1471802179-2886-1-git-send-email-xypron.glpk@gmx.de