Changed name of rd_pick_intra4x4mby_modes

The function name rd_pick_intra4x4mby_modes is confusing, so
I changed it to rd_pick_intra_sub_8x8_y_modes to better
reflect what the function does. Also added const qualifiers
to some of the input parameters and removed camel-case.

Change-Id: I23d53d4c7af5d79ed8a471acd59a09bbb47add39