Previously, segments using SEG_LVL_REF_FRAME were unable to signal the choices
of GOLDEN = 4, BWDREF = 5, ALTREF2 = 6 and ALTREF = 7, as
seg_feature_data_max[SEG_LVL_REF_FRAME] was set to 3. This patch increases the
value to 7 to account for these options.

BUG=aomedia:951

Change-Id: I9732fa2be96ead2d4b6efdbce34a92e43c7dd04e