drm/i915: Use atomic scaling_mode instead of panel.fitting_mode

The first step in converting connector properties to atomic is
wiring up the atomic state. We're still not completely supoprting
the scaling mode in the atomic case, but this is the first step
towards it.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170501133804.8116-4-maarten.lankhorst@linux.intel.com