drm: Extract drm_atomic_state_helper.[hc]

We already have a separate overview doc for this, makes sense to
untangle it from the overall atomic helpers.

v2: Rebase

v3: Rebase more.

Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004202446.22905-4-daniel.vetter@ffwll.ch