For SoC's having software designed cursor plane,
should be treated differently than hardware cursor planes.

The DRM core initializes cursor plane by default with
legacy_cursor_update set.

Hence legacy_cursor_update can be use effectively
to handle software cursor planes' update and atomicity
functionalities.

This patch uses this variable to decide in the atomic_check
to whether add a requested plane to the list of affected planes or
not, hence fixing the issue of co-existence of MPO, i.e,
setting of available hardware planes like underlay and
updation of cursor planes as well.

Without this patch when underlay is set from user space,
only blank screen with backlight is visible.
Signed-off-by: NShirish S <shirish.s@amd.com>
Reviewed-by: NHarry Wentland <Harry.Wentland@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>