drm/atomic: Handle picture_aspect_ratio in atomic core

This is only used in i915, which had used its own non-atomic way to
deal with the picture aspect ratio. Move selected aspect_ratio to
atomic state and use the atomic state in the affected i915 connectors.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170501133804.8116-2-maarten.lankhorst@linux.intel.com


[mlankhorst: taomic -> atomic thanks to Manasi's input]
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>