drm/object: Add drm_object_property_get_default_value() function

Some functions to create properties (drm_plane_create_zpos_property or
drm_plane_create_color_properties for example) will ask for a range of
acceptable value and an initial one.

This initial value is then stored in the values array for that property.

Let's provide an helper to access this property.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220221095918.18763-7-maxime@cerno.tech