drm: Make drm.h uapi header safe for C++

virtual is a protected keyword in C++ and can't be used at all. Ugh.

This aligns the kernel versions of the drm headers with the ones in
libdrm.

v2: Also annote with __user, as request by Emil&Ilia.

Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459350753-18320-1-git-send-email-daniel.vetter@ffwll.ch