drm/kms-helpers: Extract drm_modeset_helper.[hc]

While reviewing docs I spotted that we have a few functions that
really just don't fit into their containing helper library section.
Extract them and shovel them all into a new library for random one-off
aux stuff.

v2: Remove wrongly added files for real.

Cc: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471034937-651-3-git-send-email-daniel.vetter@ffwll.ch