Change two similar structs, FWD_TXFM_PARAM and INV_TXFM_PARAM,
into a common struct: TxfmParam. Its definition is moved to
aom_dsp/txfm_common.h to simplify dependency.

This change is made so that, in later changes of the LGT
experiment, functions requiring FWD_TXFM_PARAM and
INV_TXFM_PARAM, such as get_fwd_lgt4 and get_inv_lgt4, can
also be unified.

Change-Id: I756b0176a02314005060adbf8e62386f10eeb344