avcenc: Allow intra mode evaluation during fast skip

For static content, skip and intra modes often compete with each other
neck and neck during mode evaluation. Upon fast skip detection, instead of
skipping all further mode analysis, disable motion search and allow intra
analysis.

This change applies to all presets except FASTEST.

Average bdrate improves by 1%.

Test: Tested on clips of different resolutions.

Change-Id: Ia0aabca5c1dcad03359bebaf071b7be27898e735