drm/i915: Use per-connector scaling mode property

None of the intel connectors can use all types of scaling modes,
so only try the ones that are possible. This is another preparation
for connectors towards conversion to atomic.

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/20170501133804.8116-5-maarten.lankhorst@linux.intel.com
[mlankhorst: Use renamed drm_connector_attach_scaling_mode_property function]