Simplify ivas_core_enc_fx()

The function ivas_core_enc_fx() differs from its FLP counterpart ivas_core_enc(). Although recent MRs have already brought these two functions closer together, significant differences still exist.

The following changes are thus proposed to simplify ivas_core_enc_fx() and bring its structure as close as possible to its FLP counterpart ivas_core_enc():

• do scalings of buffers related to one particular module in this module, instead of in the high-level function ivas_core_enc_fx()
• introduce a new function related to the scaling of buffers, which replaces the same code blocks repeatedly used in the current function ivas_core_enc_fx()
• remove dead code related to instances like

IF( EQ_16( sts[0]->element_mode, IVAS_CPE_MDCT ) )
{
   ...
   IF( NE_16( st->element_mode, IVAS_CPE_MDCT ) )
   {
        ... dead code here ...
   }
   ...
}