Remove opaque check in preparing dirty region

Since preserve swap is enabled, we need to clear the color buffer
when the scissor rect is prepared for a new process. This prevents
garbage being present from the previous process in the color buffer.

@Topic::CR952255:Undefined region corruption fix
@Buganizer: 20151458
@Upstreaming: https://android-review.googlesource.com/#/c/146272/
CRs-Fixed: 952255

Change-Id: Icd12ae388077b8c9ed329c37314e896500078543