drm/atomic: Add support for custom scaling mode properties, v2

drm/atomic: Add support for custom scaling mode properties, v2

Some connectors may not allow all scaling mode properties, this function will allow
creating the scaling mode property with only the supported subset. It also wires up
this state for atomic.

This will make it possible to convert i915 connectors to atomic.

Changes since v1:
 - Add DRM_MODE_PROP_ENUM flag to drm_property_create
 - Use the correct index in drm_property_add_enum.
 - Add DocBook for function (Sean Paul).
 - Warn if less than 2 valid scaling modes are passed.
 - Remove level of indent. (Sean Paul)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170501133804.8116-3-maarten.lankhorst@linux.intel.com
[mlankhorst: Rename function, fix docbook issues]
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>