drm/radeon/atom: initialize more atom interpretor elements to 0

The ProcessAuxChannel table on some rv635 boards assumes
the divmul members are initialized to 0 otherwise we get
an invalid fb offset since it has a bad mask set when
setting the fb base.  While here initialize all the
atom interpretor elements to 0.

Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=60639

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org