Fix mode selection bug when ARNR filtering disabled

When ARNR filtering is disabled, by setting
arnr_max_frames=0, mode_skip_mask was being set to
-1 for the ARF frame resulting in no mode being
selected for the block.

The intent is to restrict the reference frame to the
previous ARF frame and the mode to one of ZEROMV,
NEARMV or NEARESTMV.

Change-Id: Ifc3920b153142cd01d422910c94d2f20ffb6f129