drm: Add drm/drm_util.h header file

We have a bunch of neat little macros all over the place which should
move to kernel.h. But some of them died in bikesheds on lkml, and we
need a decent home for them.

Start out by moving the for_each_if macro there.

v2: Rename to drm_util.h instead (Dave&Sean)

Cc: Sean Paul <seanpaul@chromium.org>
Acked-by: Sean Paul <seanpaul@chromium.org>
Cc: Dave Airlie <airlied@gmail.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180905135711.28370-1-daniel.vetter@ffwll.ch