hwc: Fix external display position calculation

YUV layer display positions are mapped to primary resolution
[e.g. 1920x1080] when MDP downscale is enabled, that needs to be
transformed to the original external resoltion [e.g. 1280x720].
This transformation is not taken care for 4kx2k YUV use case.

Change-Id: Ifa60773bf9fcd18676a3f05cd72a603beb165733
1 file changed