drm/amd/display: fix cursor positioning for multiplane cases

[Why]
Cursor position needs to take into account plane scaling as well.

[How]
Translate cursor coords from stream space to plane space.

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Acked-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>