Fix intra prediction ref selection in skip_encode

Fix intra prediction ref selection in skip_encode

This commit fixes the intra prediction reference source selection
in the settings of skip_encode. Use original boundary pixels as
prediction reference, when the inverse transform and reconstruction
are skipped in the per block size rate-distortion optimization loop.

Change-Id: I36081aa30aa46e203e0e6f4e8a420fd08269469a