The get_y_mode function, is superfluous, not used consistently, and requires a
useless block_idx parameter than gets pass around a lot inside the
codebase for no apparent reason.

The block parameter is misleading, as it could cause people to think all these
functions actually use this value.

Change-Id: I7ae0a8d1282c009b9114c83771cce10f5c2ee397