drm/atomic: Drop helper include from drm_atomic.c

Core code should never have to look at helper stuff, to make sure that
all helper code is 100% optional and can be overriden.

Cc: Gustavo Padovan <gustavo@padovan.org>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170630073921.2345-1-daniel.vetter@ffwll.ch