SF: accommodate 1.2 format in sampling code

Adds a cast to accommodate the use of 1.2 based pixel format
in the display sampling engine code.

Test: build
Test: vts, pts
Fixes: 122943743

This was the culprit in build-breaking topic. This branch and
Change-Id: Icf88642697b0167e5ba4a2dae7470d01c6220c15 had a silent
merge conflict that escaped TH notice. b/123404649 for full details.
This reverts commit b04e9b63.

Change-Id: Ic0946a261ffb73355909d22f972dc4de96088a6e