Cleanup inside vp9_decodemv.c.

Adding read_skip_coeff function. Renaming decode_mv to read_mv for
consistency with another function names. Removing redundant function
arguments. Renaming kfread_modes to read_intra_mode_info, read_mb_modes_mv
to read_inter_mode_info, vp9_decode_mb_mode_mv to vp9_read_mode_info,
vp9_decode_mode_mvs_init to vp9_prepare_read_mode_info. Inlining function
mb_mode_mv_init inside vp9_prepare_read_mode_info.

Change-Id: Ifee05d333da4cd331d4aff40ce41ccd9b70e494a