Fix force xrgb 8888

19c47140 introduced a regression if
there are two gbm backend and the backend fail to choose drm format.

This fix does two things:
1. Current buffer format should not be reset after create new buffer,
   otherwise current.format may just be empty after resetOutput.
2. force xrgb 8888 need to be set on the primary backend.

BUG: 448790


(cherry picked from commit e64f60ce)