Refactor intra filter type context fetch function

Factor out common codes from vp10_get_pred_context_intra_interp().
This prevents a potential invalid access of pointers xd->left_mbmi
and xd->above_mbmi.

The coding statistics are identical.

Change-Id: I72dbf9380da7359b997bbe925010faab8e9e7f8d
1 file changed