drm/radeon: fix handling of variable sized arrays for router objects

The table has the following format:

typedef struct _ATOM_SRC_DST_TABLE_FOR_ONE_OBJECT         //usSrcDstTableOffset pointing to this structure
{
  UCHAR               ucNumberOfSrc;
  USHORT              usSrcObjectID[1];
  UCHAR               ucNumberOfDst;
  USHORT              usDstObjectID[1];
}ATOM_SRC_DST_TABLE_FOR_ONE_OBJECT;

usSrcObjectID[] and usDstObjectID[] are variably sized, so we
can't access them directly.  Use pointers and update the offset
appropriately when accessing the Dst members.

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