Adjust gm parameter computation to avoid mismatch

The gm parameters need to have WARPED_PRECISION_BITS precision
until they are written to the bitstream because functions in
reconinter use these parameters before they are written to
the bitstream. Previously, the parameters weren't being converted
to WARPED_PRECISION_BITS until they were read from the bitstream
which causes an encode/decode mismatch.

Change-Id: I31e76e9d6f7d24df21af287a72f8c01f1997304d